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

* feat: Add Breadth First Search (graph algorithm)

* feat: Add Depth First Search (graph algorithm)

Graphs/BFS.py

@@ -0,0 +1,39 @@
+"""pseudo-code"""
+
+"""
+BFS(graph G, start vertex s):
+// all nodes initially unexplored
+mark s as explored
+let Q = queue data structure, initialized with s
+while Q is non-empty:
+    remove the first node of Q, call it v
+    for each edge(v, w):  // for w in graph[v]
+        if w unexplored:
+            mark w as explored
+            add w to Q (at the end)
+
+"""
+
+import collections
+
+
+def bfs(graph, start):
+    explored, queue = set(), [start]  # collections.deque([start])
+    explored.add(start)
+    while queue:
+        v = queue.pop(0)  # queue.popleft()
+        for w in graph[v]:
+            if w not in explored:
+                explored.add(w)
+                queue.append(w)
+    return explored
+
+
+G = {'A': ['B', 'C'],
+     'B': ['A', 'D', 'E'],
+     'C': ['A', 'F'],
+     'D': ['B'],
+     'E': ['B', 'F'],
+     'F': ['C', 'E']}
+
+print(bfs(G, 'A'))

Graphs/DFS.py

@@ -0,0 +1,36 @@
+"""pseudo-code"""
+
+"""
+DFS(graph G, start vertex s):
+// all nodes initially unexplored
+mark s as explored
+for every edge (s, v):
+    if v unexplored:
+        DFS(G, v)
+"""
+
+
+def dfs(graph, start):
+    """The DFS function simply calls itself recursively for every unvisited child of its argument. We can emulate that
+     behaviour precisely using a stack of iterators. Instead of recursively calling with a node, we'll push an iterator
+      to the node's children onto the iterator stack. When the iterator at the top of the stack terminates, we'll pop
+       it off the stack."""
+    explored, stack = set(), [start]
+    explored.add(start)
+    while stack:
+        v = stack.pop()  # the only difference from BFS is to pop last element here instead of first one
+        for w in graph[v]:
+            if w not in explored:
+                explored.add(w)
+                stack.append(w)
+    return explored
+
+
+G = {'A': ['B', 'C'],
+     'B': ['A', 'D', 'E'],
+     'C': ['A', 'F'],
+     'D': ['B'],
+     'E': ['B', 'F'],
+     'F': ['C', 'E']}
+
+print(dfs(G, 'A'))