Fix Non Recursive Depth First Search

Author: marcoscannabrava

graphs/depth_first_search.py

@@ -19,7 +19,7 @@ def depth_first_search(graph: Dict, start: str) -> Set[int]:
     """Depth First Search on Graph
 
        :param graph: directed graph in dictionary format
-       :param vertex: starting vectex as a string
+       :param vertex: starting vertex as a string
        :returns: the trace of the search
        >>> G = { "A": ["B", "C", "D"], "B": ["A", "D", "E"],
        ... "C": ["A", "F"], "D": ["B", "D"], "E": ["B", "F"],
@@ -32,13 +32,15 @@ def depth_first_search(graph: Dict, start: str) -> Set[int]:
        True
     """
     explored, stack = set(start), [start]
+
     while stack:
         v = stack.pop()
-        # one 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)
+        explored.add(v)
+        # Differences from BFS:
+        # 1) pop last element instead of first one
+        # 2) add adjacent elements to stack without exploring them
+        for adj in reversed(graph[v]):
+            if adj not in explored: stack.append(adj)
     return explored