Correct the wrong iterative DFS implementation (TheAlgorithms#867)

* Update DFS.py

* Update DFS.py

Author: ZJUGuoShuai

graphs/DFS.py

@@ -18,10 +18,15 @@ def dfs(graph, start):
     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
+        v = stack.pop()  # one difference from BFS is to pop last element here instead of first one
+        
+        if v in explored:
+            continue
+
+        explored.add(v)
+
         for w in graph[v]:
             if w not in explored:
-                explored.add(w)
                 stack.append(w)
     return explored