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Difference between BFS and DFS

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  • Difficulty Level : Easy
  • Last Updated : 12 Sep, 2022
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Breadth-First Search:
BFS, Breadth-First Search, is a vertex-based technique for finding the shortest path in the graph. It uses a Queue data structure that follows first in first out. In BFS, one vertex is selected at a time when it is visited and marked then its adjacent are visited and stored in the queue. It is slower than DFS. 
Example:

 Input:
        A
       / \
      B   C
     /   / \
    D   E   F

Output: 

A, B, C, D, E, F

Depth First Search:
DFS, Depth First Search, is an edge-based technique. It uses the Stack data structure and performs two stages, first visited vertices are pushed into the stack, and second if there are no vertices then visited vertices are popped. 
Example: 

  Input:
        A
       / \
      B   D
     /   / \
    C   E   F

Output: 

A, B, C, D, E, F

BFS vs DFS

S. No.       ParametersBFSDFS
1.Stands forBFS stands for Breadth First Search.DFS stands for Depth First Search.
2.Data StructureBFS(Breadth First Search) uses Queue data structure for finding the shortest path.DFS(Depth First Search) uses Stack data structure.
3.DefinitionBFS is a traversal approach in which we first walk through all nodes on the same level before moving on to the next level.  DFS is also a traversal approach in which the traverse begins at the root node and proceeds through the nodes as far as possible until we reach the node with no unvisited nearby nodes.
4.TechniqueBFS can be used to find a single source shortest path in an unweighted graph because, in BFS, we reach a vertex with a minimum number of edges from a source vertex. In DFS, we might traverse through more edges to reach a destination vertex from a source.
5.Conceptual DifferenceBFS builds the tree level by level.DFS builds the tree sub-tree by sub-tree.
6.Approach usedIt works on the concept of FIFO (First In First Out). It works on the concept of LIFO (Last In First Out).
7.Suitable forBFS is more suitable for searching vertices closer to the given source.DFS is more suitable when there are solutions away from source.
8.Suitable for Decision TreestheirwinningBFS considers all neighbors first and therefore not suitable for decision-making trees used in games or puzzles.DFS is more suitable for game or puzzle problems. We make a decision, and the then explore all paths through this decision. And if this decision leads to win situation, we stop.
9.Time ComplexityThe Time complexity of BFS is O(V + E) when Adjacency List is used and O(V^2) when Adjacency Matrix is used, where V stands for vertices and E stands for edges.The Time complexity of DFS is also O(V + E) when Adjacency List is used and O(V^2) when Adjacency Matrix is used, where V stands for vertices and E stands for edges.
10.Visiting of Siblings/ ChildrenHere, siblings are visited before the children.Here, children are visited before the siblings.
11.Removal of Traversed NodesNodes that are traversed several times are deleted from the queue. The visited nodes are added to the stack and then removed when there are no more nodes to visit.
12.BacktrackingIn BFS there is no concept of backtracking. DFS algorithm is a recursive algorithm that uses the idea of backtracking
13.ApplicationsBFS is used in various applications such as bipartite graphs, shortest paths, etc.DFS is used in various applications such as acyclic graphs and topological order etc.
14.Memory BFS requires more memory. DFS requires less memory. 
15.OptimalityBFS is optimal for finding the shortest path.DFS is not optimal for finding the shortest path.
16.Space complexityIn BFS, the space complexity is more critical as compared to time complexity.DFS has lesser space complexity because at a time it needs to store only a single path from the root to the leaf node.
17.SpeedBFS is slow as compared to DFS.DFS is fast as compared to BFS.
18.When to use?When the target is close to the source, BFS performs better. When the target is far from the source, DFS is preferable. 

Please also see BFS vs DFS for Binary Tree for the differences for a Binary Tree Traversal.
 

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