Commit 1cbbd5f

kHarshitpoyea

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Added BFS and DFS (graph algorithms) (TheAlgorithms#408)

* feat: Add Breadth First Search (graph algorithm) * feat: Add Depth First Search (graph algorithm)

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Lines changed: 39 additions & 0 deletions

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	`1`	`+"""pseudo-code"""`
	`2`	`+`
	`3`	`+"""`
	`4`	`+BFS(graph G, start vertex s):`
	`5`	`+// all nodes initially unexplored`
	`6`	`+mark s as explored`
	`7`	`+let Q = queue data structure, initialized with s`
	`8`	`+while Q is non-empty:`
	`9`	`+ remove the first node of Q, call it v`
	`10`	`+ for each edge(v, w): // for w in graph[v]`
	`11`	`+ if w unexplored:`
	`12`	`+ mark w as explored`
	`13`	`+ add w to Q (at the end)`
	`14`	`+`
	`15`	`+"""`
	`16`	`+`
	`17`	`+import collections`
	`18`	`+`
	`19`	`+`
	`20`	`+def bfs(graph, start):`
	`21`	`+ explored, queue = set(), [start] # collections.deque([start])`
	`22`	`+ explored.add(start)`
	`23`	`+ while queue:`
	`24`	`+ v = queue.pop(0) # queue.popleft()`
	`25`	`+ for w in graph[v]:`
	`26`	`+ if w not in explored:`
	`27`	`+ explored.add(w)`
	`28`	`+ queue.append(w)`
	`29`	`+ return explored`
	`30`	`+`
	`31`	`+`
	`32`	`+G = {'A': ['B', 'C'],`
	`33`	`+ 'B': ['A', 'D', 'E'],`
	`34`	`+ 'C': ['A', 'F'],`
	`35`	`+ 'D': ['B'],`
	`36`	`+ 'E': ['B', 'F'],`
	`37`	`+ 'F': ['C', 'E']}`
	`38`	`+`
	`39`	`+print(bfs(G, 'A'))`

Lines changed: 36 additions & 0 deletions

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	`1`	`+"""pseudo-code"""`
	`2`	`+`
	`3`	`+"""`
	`4`	`+DFS(graph G, start vertex s):`
	`5`	`+// all nodes initially unexplored`
	`6`	`+mark s as explored`
	`7`	`+for every edge (s, v):`
	`8`	`+ if v unexplored:`
	`9`	`+ DFS(G, v)`
	`10`	`+"""`
	`11`	`+`
	`12`	`+`
	`13`	`+def dfs(graph, start):`
	`14`	`+ """The DFS function simply calls itself recursively for every unvisited child of its argument. We can emulate that`
	`15`	`+ behaviour precisely using a stack of iterators. Instead of recursively calling with a node, we'll push an iterator`
	`16`	`+ to the node's children onto the iterator stack. When the iterator at the top of the stack terminates, we'll pop`
	`17`	`+ it off the stack."""`
	`18`	`+ explored, stack = set(), [start]`
	`19`	`+ explored.add(start)`
	`20`	`+ while stack:`
	`21`	`+ v = stack.pop() # the only difference from BFS is to pop last element here instead of first one`
	`22`	`+ for w in graph[v]:`
	`23`	`+ if w not in explored:`
	`24`	`+ explored.add(w)`
	`25`	`+ stack.append(w)`
	`26`	`+ return explored`
	`27`	`+`
	`28`	`+`
	`29`	`+G = {'A': ['B', 'C'],`
	`30`	`+ 'B': ['A', 'D', 'E'],`
	`31`	`+ 'C': ['A', 'F'],`
	`32`	`+ 'D': ['B'],`
	`33`	`+ 'E': ['B', 'F'],`
	`34`	`+ 'F': ['C', 'E']}`
	`35`	`+`
	`36`	`+print(dfs(G, 'A'))`

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