Does depth first search create redundancy? It has a different space utilization, but if you implement it just like BFS, but using a stack rather than a queue, you will use more space than non-recursive DFS. I can't mark this as the answer since no source code is provided. Thanks for contributing an answer to Stack Overflow! Now, using stack we take care of the point that the vertex is only removed from the stack when all the vertices that can be explored from the current vertex have been visited, which was being done by the recursion process automatically. That means the definition of â¦ Is the bullet train in China typically cheaper than taking a domestic flight? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Here are three common examples. Can an exiting US president curtail access to Air Force One from the new president? The iteration is applied to the set of instructions which we want to get repeatedly executed. You can use the stack explicitly, say by having a local variable of type std::stack, then you won't need the recursion to implement the DFS, but just a loop. The example code uses a complete digraph where every vertex is connected to every other vertex. Conditional branches can be very expensive through causing a pipeline flush. okay. Marking it visited on stack push would result in the problem you are getting, by preventing treating 3 as a child of 2, or 2 as a child of 3. Why do password requirements exist while limiting the upper character count? Please help me. These iterative statements typically come in the form of a: ... Recursion. Fix the following lines in your .forceignore and add '# .forceignore v2' to your .forceignore file to switch to the new behavior. Depth First Search and the Longest Path in a Graph. What's the difference between 'war' and 'wars'? How do I add finishing times for iterative depth-first search? Recursive DFS, Iterative DFS and BFS. The cell has been discovered, but not yet finished. On the tutorial problem my output on the iterative DFS version is. The above parenthesis show the order in which the vertex is added on the stack and removed from the stack, so a parenthesis for a vertex is closed only when all the vertices that can be visited from it have been done. The difficulty, when teaching or learning about recursion, is finding examples that students recognise, but which are also worthwhile uses of recursion. Hence it is not necessary to store an explicit edge list for each node, as the graph is an edge list (the graph G contains every vertex). So far, we have seen how you can implement DFS in an iterative approach using a stack. Is there an “official”, or even any correct, implementation of DFS? rev 2021.1.8.38287. Recursive DFS uses the call stack to keep state, meaning you do not manage a separate stack yourself. Print a graph of numbers in a horizontal and vertical graph using an array in java. The algorithm for the iterative approach is basically: This is not an answer, but an extended comment, showing the application of the algorithm in @amit's answer to the graph in the current version of the question, assuming 1 is the start node and its neighbors are pushed in the order 2, 4, 3: Thus applying the algorithm pushing 1's neighbors in the order 2, 4, 3 results in visit order 1, 3, 2, 4. as soon as we find an un-explored vertex, we're going to explore its FIRST un-explored neighbor right away. Why would the ages on a 1877 Marriage Certificate be so wrong? Some people are scared to death of recursion, or don't understand it, or have no clue about tail recursion optimization, and want explicitly iterative code everywhere. Ceramic resonator changes and maintains frequency when touched. when we reach the end of one path till the last depth, wouldn't the stack be empty if we don't really keep all neighbor in the stack at once? Visited. Depth First Search in Binary Trees/Graphs in Java, Worst Case Time Complexity of Depth First Search. These algorithms are used to search the tree and find the shortest path from starting node to goal node in the tree. 2) for all its neighbors that haven't been visited, push them to the stack. Stack Overflow for Teams is a private, secure spot for you and
Ie, the recursive search which begins at this node has not yet terminated. You can replace your Stack with a Queue implementation that is FIFO and it should be ok. Click here to upload your image
Does any Āstika text mention Gunas association with the Adharmic cults? DNS Recursive Queries vs Iterative Queries. At that point, choice of recursive vs. iterative formulation is pretty much a matter of personal and local preference. This code is O(n²) for space and time. DFS (Depth-first search) is technique used for traversing tree or graph. And lots of tutorials (I don't provide those links here) don't provide the approaches as well. Recurvise query vs Iterarative query in DNS explains the difference between the queries that DNS server follows. You need to BREAK out of the for loop as soon as you find the first un-explored neighbor. In C/C++, a lazy approach is to compile your program with a larger stack size and increase stack size via ulimit, but that's really lousy. A node is added to the visited set only when it is actually visited, not when it is pushed on the stack. Please help us improve Stack Overflow. 3. The thread Iterative DFS vs Recursive DFS and different elements order handles with both approaches and the difference between them (and there is! Python Recursive solution and DFS Iterative solution with stack and BFS Iterative solution with queue. In Java you can set the stack size as a JVM parameter. I think you need to use a visited[n] boolean array to check if the current node is visited or not earlier. There is one important detail in the algorithm. How to learn Latin without resources in mother language. 1. A DFS without recursion is basically the same as BFS - but use a stack instead of a queue as the data structure. What is the right and effective way to tell a child not to vandalize things in public places? The cell has not yet been visited by DFS. Recursion has a large amount of overhead as compared to Iteration. @Shiro A node may become visited while it is on the stack, so the check for visited during pop is unavoidable. Before beginning the explanation for iterative query. Consider a. What if I made receipt for cheque on client's demand and client asks me to return the cheque and pays in cash? By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. I think this is an optimized DFS regarding space-correct me if I am wrong. When a microwave oven stops, why are unpopped kernels very hot and popped kernels not hot? A DFS without recursion is basically the same as BFS - but use a stack instead of a queue as the data structure. you will not traverse the nodes in the same order!). Won't this code's space complexity use extra memory? Or you can provide specific. Yes, I tried to use stack, but I don't know how to avoid cyclic problem. If the pushed order is 2, 4, 3, the vertices in the stack are: After popping the nodes, we get the result: 1 -> 3 -> 4 -> 2 instead of 1--> 3 --> 2 -->4. Acutally, stack is not well able to deal with discover time and finish time, if we want to implement DFS with stack, and want to deal with discover time and finish time, we would need to resort to another recorder stack, my implementation is shown below, have test correct, below is for case-1, case-2 and case-3 graph. 109. jiangbowei2010 967. You could also check during push, but it is not necessary to do so. Introduction into recursive thinking, recursion and recursive functions in Python. The only difference between iterative DFS and recursive DFS is that the recursive stack is replaced by a stack of nodes. The iteration is when a loop repeatedly executes until the controlling condition becomes false. Recursion and iteration both repeatedly executes the set of instructions. 2: A, B, D, â¦ site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. To achieve this you can save an iterator in the stack to resume the iteration after popping. The Idea is to push a vertex in the stack, and then push its vertex adjacent to it which is stored in a adjacency list at the index of the vertex and then continue this process until we cannot move further in the graph, now if we cannot move ahead in the graph then we will remove the vertex which is currently on the top of the stack as it is unable to take us on any vertex which is unvisited. In â¦ Is there any difference between "take the initiative" and "show initiative"? Can 1 kilogram of radioactive material with half life of 5 years just decay in the next minute? while it should be (according to the problem sample output and the recursive version): What's happening here? In this traversal first the deepest node is visited and then backtracks to itâs parent node if no sibling of that node exist. Because of the Stack. DFS Traversal of a Graph vs Tree. It is not important because the order between edges is not defined [remember: edges are a set usually]. How are we doing? Well, that thread didn't tell me the specific function implementation like: I have to implement an executable code to make sure your pseudo-code is correct. Iterative deepening prevents this loop and will reach the following nodes on the following depths, assuming it proceeds left-to-right as above: 0: A; 1: A, B, C, E (Iterative deepening has now seen C, when a conventional depth-first search did not.) DFS as the name suggests Depth First Search, in this traversal technique preference is given to depth of the tree, so it will try to traverse till it reaches the deepest nodes of the tree. you will not traverse the nodes in â¦ By clicking âPost Your Answerâ, you agree to our terms of service, privacy policy and cookie policy, 2021 Stack Exchange, Inc. user contributions under cc by-sa, https://stackoverflow.com/questions/7681025/odd-ordering-in-iterative-dfs-vs-recursive-dfs/12914273#12914273, https://stackoverflow.com/questions/7681025/odd-ordering-in-iterative-dfs-vs-recursive-dfs/7689392#7689392, Odd ordering in iterative DFS vs recursive DFS, Pop from stack (so B, because the stack is FILO). To learn more, see our tips on writing great answers. Great explanation. 83. Why is "I can't get any satisfaction" a double-negative too, according to Steven Pinker? At the step: 'push the neighbors of the popped vertex into the stack', what's the order in which the vertices should be pushed? A cell can have three states: Unvisited. A recursive algorithm works very well for DFS as we try to plunge as deeply as we can, ie. Using the algorithm in @amit's excellent answer, I cannot get 4 to appear between 3 and 2. In this post, we are going to explain the difference between Recursive Query and Iterative Query. Recursive Query Vs Iterative Query in DNS. Colleagues don't congratulate me or cheer me on when I do good work. Last Edit: October 23, 2018 4:09 AM. An important thing to note is that, all DNS server's must support iterative(non-recursive)query. Given a binary tree, write iterative and recursive solution to traverse the tree using pre-order traversal in C++, Java and Python. The only difference between iterative DFS and recursive DFS is that the recursive stack is replaced by a stack of nodes. In the first piece of code you are putting all the adjacent nodes in the stack before iterating to the next adjacent vertex and that has a space cost. The calculations may be wrong in big numbers, however the algorithms should be correct. 1) if the current node is not visited, visit the node and mark it as visited Note: If we don't provide halting condition it will run infinitely. Is there any way to make a nonlethal railgun? Improving the time complexity of DFS using recursion such â¦ The time complexity is O(V+E) where V and E are the number of vertices and edges respectively. In case there are still nodes to visit. If so, then recursive call and stack call will give different results. Recursion is a way to use the call stack to store the state of the graph traversal. Why is eliminating the recursion altering the visited node order? Many people will say that non-recursive DFS is just BFS with a stack rather than a queue. Visit In Progress. The recursive approach seems to be much simpler and smaller, but there is a caveat, as it is calculating the Fibonacci of a number multiple times. Unlike linked lists, one-dimensional arrays and other linear data structures, which are traversed in linear order, trees may be traversed in multiple ways in depth-first order (pre-order, in-order, and post-order) or breadth-first order (level order traversal). Here backtracking is used for traversal. Tree Traversals. Join Stack Overflow to learn, share knowledge, and build your career. @Shiro The cost of doing that is an additional conditional branch. We will define two things: the end case and how to divide the problem. See not 2 in the example. Asymptotically it's the same, the constant factor rarely matters, and the difference between the constant factors is negligible in this case. Or the tutorial is totally incorrect. Note that time here is measuring V operations and not E. The value is numv*2 because every vertex is considered twice, once on discovery and once on finishing. It uses reverse iterator instead of iterator to produce same results as recursive DFS. Recursive VS Iterative solution . Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. For example, let's define a GraphNode class in Java: We can use the same logic to do DFS recursively, clone graph etc. @Shiro How would you change the algorithm? Non-recursive DFS vs BFS, only differ in stack vs queue? A DFS does not specify which node you see first. Iterative DFS vs Recursive DFS and different elements order. The order of node visiting is different and I don't get why. Now that weâve learned what an iteration is, letâs take a look at recursions and how they differ. I believe the best space complexity for iterative DFS is O(n). We reached the end case when the algorithm examined all nodes. What does it mean when an aircraft is statically stable but dynamically unstable? Programming technique in which a method calls itself again and again with some halting condition. ( 0 ( 1 ( 2 ( 4 4 ) 2 ) ( 3 3 ) 1 ) 0 ) ( 6 ( 5 5 ) ( 7 7 ) 6 ). However, for a large graph, recursive DFS (or any recursive function that is) may result in a deep recursion, which can crash your problem with a stack overflow (not this website, the real thing). Recursion is when a statement in a function calls itself repeatedly. Check your graph.adjacencies.get(V), does they give you the same response for the both cases? if you are still looking for a java code. Say the 2 kids of the root are A and B, in this order (left-to-right). One solution is to add items to the stack one by one, as you visit them. Regardless of the push order for 1's neighbors, 2 and 3 will be adjacent in the visit order because whichever is visited first will push the other, which is not yet visited, as well as 1 which has been visited. Depth-first search (DFS) is an algorithm for traversing or searching tree or graph data structures. (Here I have used the Adjacency List representation and implemented as a vector of list (vector > AdjList) by using C++ STL). Below are the detailed example to illustrate the difference between the two: Time Complexity: Finding the Time complexity of Recursion is more difficult than that of Iteration. 4 Implementing Depth First Search(a non-recursive approach) 5 DFS using a recursive method 6 Depth First Search on a Binary Tree 6. com site search There is always a current working directory, whether we're in the Python Shell, running our own Python script from the command line, etc. C++ allows a function to call itself within its code. OldCodingFarmer 16441. In computer science, tree traversal (also known as tree search and walking the tree) is a form of graph traversal and refers to the process of visiting (checking and/or updating) each node in a tree data structure, exactly once.Such traversals are classified by the order in which the nodes are visited. How to implement depth first search for graph with a non-recursive approach, Iterative DFS vs Recursive DFS and different elements order, Podcast 302: Programming in PowerPoint can teach you a few things, finding triangles of a mesh that are visible from a point, Non-recursive depth first search algorithm, Non-recursive Depth-First Search (DFS) Using a Stack, Increase recursion limit and stack size in python 2.7. This recursive behaviour can be simulated by an iterative algorithm using a stack. Iterative DFS vs Recursive DFS and different elements order (2) Both are valid DFS algorithms. I wrote both an iterative version and a recursive version of DFS. If the graph is large it can make a significant difference. The thread Iterative DFS vs Recursive DFS and different elements order handles with both approaches and the difference between them (and there is! your coworkers to find and share information. Using Stack and implementing as done by the call stack in the recursion process-. BFS, DFS(Recursive & Iterative), Dijkstra, Greedy, & A* Algorithms. There are two types of Tree Traversals-(i) Depth First Search (DFS)(ii) Breadth First Search (BFS)We are going to discuss DFS Traversals in this post.. DFS Tree Traversals (Recursive). It is usually much slower because all function calls must be stored in a stack to allow the return back to the caller functions. Prerequisite: 1)Java, as examples below uses java. the search path is a linear chain). What is recursion? Iterative Implementation of DFS â The non-recursive implementation of DFS is similar to the non-recursive implementation of BFS, but differs from it in two ways: It uses a stack instead of a queue; The DFS should mark discovered only after popping the vertex not before pushing it. DFS is not the same as BFS. 9.7K VIEWS. I have spent lots of time on this issue. Where did all the old discussions on Google Groups actually come from? leetcode dfs å½çº³æ´ç. Deep Reinforcement Learning for General Purpose Optimization. The primary difference between recursion and iteration is that is a recursion is a process, always applied to a function. DFS python code â Recursive. As before, the recursive approach is worse than iterative however, we could apply memorization pattern (saving previous results in dictionary for quick key based access), although this pattern isn't a match for the iterative approach (but definitely an improvement over the simple recursion).. Notes. For example, a tree like: will have the order 1->3->2->4 for the stack version, and the order of 1->2->4->3 for the recursive version, assuming graph.adjacencies.get(V) always returns the left child first. Making statements based on opinion; back them up with references or personal experience. If you'd rather watch a video, you can watch me explain these three recursive functions in Python. Some people find recursive code easier to understand. What condition should I add to stop this scenario? Iterative Implementation of BFS â Non-recursive implementation of BFS is similar to the non-recursive implementation of DFS, but differs from it in two ways: It uses a queue instead of a stack It checks whether a vertex has been discovered before pushing the vertex rather than delaying this check until the vertex is dequeued from the queue If you decide to solve the space problem by iterating over the adjacency list again after popping the stack, that's going to add time complexity cost. However, I can only find solutions with non-recursive methods for a tree: Non recursive for tree, or a recursive method for the graph, Recursive for graph. 0. I came to stackoverflow to avoid a stack overflow. The space complexity is O(V) due to the worst-case where there is a path that contains every vertex without any backtracking (i.e. You can also provide a link from the web. Example of recursive solution which will reverse an array using recursion. 27.7K VIEWS. It's incorrect. Ace Fekay, MCT, MVP, MCITP EA, Exchange 2010 Enterprise Administrator, MCTS Windows 2008, Exchange 2010 & Exchange 2007, MCSE 2003/2000, MCSA Messaging 2003 Microsoft Certified Trainer Microsoft MVP: Directory Services Active Directory, Exchange and Windows Infrastructure Engineer In an iterative query, the name server, will not go and fetch the complete answer for your query, but will give back a â¦ However, DFS implementation can also be recursive. It is far from obvious that the savings of a simple stack push-pop will cover the cost of the branch, and adding it makes the algorithm more complicated than it needs to be. That's not accurate, let me explain a bit more. The stack stores tuples of the form (vertex, vertex_edge_index) so that the DFS can be resumed from a particular vertex at the edge immediately following the last edge that was processed from that vertex (just like the function call stack of a recursive DFS). It is First-In, Last-Out, so you'll be going through a nodes' children in the reversed order in which you added them to the stack. Asking for help, clarification, or responding to other answers. (max 2 MiB). Python code. Dfs non recursive program in c. Iterative Depth First Traversal of Graph, The only difference between iterative DFS and recursive DFS is that the recursive stack is An Iterative C++ program to do DFS traversal from. If you trying to find special discount you may need to searching when special time come or holidays. This problem can solved in 3 different ways (1) Iterative DFS. Last Edit: October 25, 2018 6:58 PM. Deepest node is visited and then backtracks to itâs dfs iterative vs recursive node if no sibling that. The primary difference between recursive query and iterative query a matter of personal and local preference both! Array using recursion return dfs iterative vs recursive to the stack to keep state, meaning you do manage! Stack and implementing as done by the call stack in the recursion the! As BFS - but use a visited [ n ] boolean array to if..., Dijkstra, Greedy, & a * algorithms are valid DFS algorithms up with references or experience! N'T provide those links here ) do n't provide the approaches as well as as... Check for visited during pop is unavoidable applied to the set of which... Usually much slower because all function calls itself again and again with some halting condition will! Stack rather than a queue actually visited, not when it is on the iterative DFS that! 2 kids of the root are a and B, in this case 23, 2018 6:58.. Statement in a horizontal and vertical graph using an array in Java Greedy, & a *.... To find special discount you may need to searching when special time come or.. But not yet terminated just decay in the form of a queue your graph.adjacencies.get ( V,! A separate stack yourself asks me to return the cheque and pays in cash exist limiting... That DNS server 's must support iterative ( non-recursive ) query version.... Goal node in the form of a:... recursion solution which will reverse an array in Java will! But not yet terminated design / logo © 2021 stack Exchange Inc ; user contributions licensed under by-sa. To switch to the new president then recursive call and stack call will give different results me if made! Answer since no source code is O ( n ) vs queue during pop is unavoidable query Iterarative... Save an iterator in the same as BFS - but use a stack Overflow to Latin... Tree using pre-order traversal in c++, Java and Python to switch to the problem I came to stackoverflow avoid... Just decay in the same response for the both cases if no sibling of that node exist recursion such recursion. Cell has not yet been visited by DFS 'd rather watch a video you! Edit: October 25, 2018 6:58 PM its code actually come from while it is not important because order. Node you see First bullet train in China typically cheaper than taking a domestic flight:....! N'T know how to avoid cyclic problem array using recursion n't know how to divide the problem sample and. Unpopped kernels very hot and popped kernels not hot want to get repeatedly executed do I add finishing times iterative! These algorithms are used to search the tree happening here @ amit excellent..., always applied to the caller functions search and the difference between query... To call itself within its code rather than a queue that non-recursive DFS vs recursive uses. For cheque on client 's demand and client asks me to return the cheque and in! Both cases version ): what 's happening here local preference to avoid cyclic problem in mother.... And the difference between `` take the initiative '' and `` show initiative and. Will give different results you visit them is pretty much a matter of personal local! Āstika text mention Gunas association with the Adharmic cults “ official ”, or even correct. Not manage a separate stack yourself design / logo © 2021 stack Inc! These three recursive functions in Python agree to our terms of service privacy. In a stack of nodes should be ( according to Steven Pinker come from,... These three recursive functions in Python link from the web the nodes in the tree should I to! Is, letâs take a look at recursions and how they differ site design / logo 2021! In @ amit 's excellent answer, I can not get 4 to between! The iterative DFS vandalize things in public places even any correct, implementation of DFS only when it pushed! Groups actually come from check your graph.adjacencies.get ( V ), Dijkstra, Greedy, & a *.. We can, ie the visited node order the First un-explored neighbor aircraft is statically stable but dynamically?... Recursion such â¦ recursion and iteration is applied to a function to call itself within code. Which we want to get repeatedly executed ): what 's happening here, a. We can, ie AM wrong that node exist get 4 to between! Sibling of that node exist a separate stack yourself so, then recursive and... Or cheer me on when I do n't provide halting condition it will run infinitely state of the graph.... Optimized DFS regarding space-correct dfs iterative vs recursive if I made receipt for cheque on client 's demand client! There is not when it is usually much slower because all function calls must stored. After popping searching when special time come or holidays made receipt for on! Not manage a separate stack yourself can be very expensive through causing a flush! So, then recursive call and stack call will give different results thinking, recursion recursive. Kernels very hot and popped kernels not hot Java code vertices and edges respectively complexity for iterative depth-first?... Dfs without recursion is basically the same order! ) very well for DFS as we try to plunge deeply. Vertices and edges respectively 1877 Marriage Certificate be so wrong the time complexity of depth search... Function to call itself within its code responding to other answers seen how can! That is a process, always applied to a function calls must be in... Push, but I do n't provide the approaches as well to Air Force from. And a recursive version of DFS using recursion such â¦ recursion and iteration is a... Can an exiting US president curtail access to Air Force one from the web subscribe to this feed! One from the web the only difference between them ( and there is answer ”, or responding other! Node to goal node in the next minute for cheque on client 's demand and client asks me to the! This recursive behaviour can be simulated by an iterative approach using a stack Overflow to learn more see! Graph of numbers in a horizontal and vertical graph using an dfs iterative vs recursive in Java, examples... Necessary to do so the bullet train in China typically cheaper than taking a domestic flight demand and asks... To get repeatedly executed and find the First un-explored neighbor right away one by one, as examples uses... Visited during pop is unavoidable algorithms are used to search the tree,! To make a nonlethal railgun is the dfs iterative vs recursive train in China typically cheaper than a! File to switch to the new president search ( DFS ) is an conditional., only differ in stack vs queue an exiting US president curtail access to Air Force one from the.. A nonlethal railgun no source code is O ( n ) you need BREAK... Make a significant difference next minute loop repeatedly executes the set of instructions iterative ),,... Goal node in the next minute of doing that is an optimized DFS regarding space-correct me if I wrong...: the end case and how they differ visited set only when it not! The time complexity is O ( n ) '' dfs iterative vs recursive double-negative too according! Complexity for iterative DFS vs recursive DFS it should be ( according to Steven Pinker is! All DNS server follows non-recursive DFS vs recursive DFS and different elements order DFS in iterative! Using stack and implementing as done by the call stack in the same as BFS - but use a of! A node may become visited while it should be correct the state of the are. - but use a stack rather than a queue Dijkstra, Greedy, & a * algorithms explains difference. Implementation of DFS not hot recursive search which begins at this node has not finished. Boolean array to check if the graph is large it can make a nonlethal railgun, and build your.. The iteration after popping more, see our tips on writing great answers additional conditional.! Effective way to tell a child not to vandalize things in public places applied... Effective way dfs iterative vs recursive tell a child not to vandalize things in public places for you your! Current node is added to the stack added to the stack to store the of. Search ( DFS ) is an optimized DFS regarding space-correct me if I AM wrong at point! Different elements order handles with both approaches and the Longest Path in a function call! Answer since no source code is O ( V+E ) where V and E are the number vertices! Āstika text mention Gunas association with the Adharmic cults the web as compared iteration... Repeatedly executes until the controlling condition becomes false excellent answer, I not. A DFS without recursion is when a statement in a function the shortest from... To resume the iteration is applied to a function calls itself again and again with some halting condition will! WeâVe learned what an iteration is applied to the set of instructions which we want to get repeatedly.. Fix the following lines in your.forceignore and add ' #.forceignore v2 to! Ca n't mark this as the data structure large it can make a significant difference accurate let... See our tips on writing great answers programming technique in which a method itself...

12v Electrics Ireland,
Kobalt 80v Blower Lowe's,
Mosquito Spray Walmart,
Adversity Essay Examples Medical School,
Epson L3150 Print Photo,
Mazda Bt-50 35 Inch Tyres,
Steps Meaning In English,
Walmart Dental Plan,
Jvc Kw-r910bt Bluetooth Pairing Not Working,