Update and rename 261. Graph Valid Tree.md to 261._Graph_Valid_Tree.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/261. Graph Valid Tree.md renamed to docs/Leetcode_Solutions/Python/261._Graph_Valid_Tree.md

@@ -68,9 +68,54 @@ class Solution:
         return len(set(find(i) for i in uf)) == 1 # fully connected
 ```
 
+> 思路 2
+******- 时间复杂度: O(V + E)******- 空间复杂度: O(V)******
 
+- DFS判断是否从起始点可以走完所有点（是否完全联通）
+- 中间碰到一个点，除了它来自的那个点(prev_v)以外，它其余的所有neghbor都应该是没有被访问过的（否则有环）
 
+beats 36.01%
 
+```python
+class Graph(object):
+    def __init__(self, vertex_cnt):
+        self.vertex_cnt = vertex_cnt
+        self.graph = collections.defaultdict(list)
+        
+    def addEdge(self, v1, v2):
+        self.graph[v1].append(v2)
+        self.graph[v2].append(v1)
+        
+    def hasCycleHelper(self, v, visited, prev_v):
+        self.visited[v] = True
+        for i in self.graph[v]:
+            if not visited[i]:
+                if self.hasCycleHelper(i, visited, v):
+                    return True
+            else:
+                if i != prev_v:
+                    return True
+        return False
+    
+    def hasCycle(self):
+        self.visited = [False] * self.vertex_cnt
+        if self.hasCycleHelper(0, self.visited, -1): 
+            return True
+        return False
+                
+        
+class Solution:
+    def validTree(self, n, edges):
+        """
+        :type n: int
+        :type edges: List[List[int]]
+        :rtype: bool
+        """
+        graph = Graph(n)
+        for node1, node2 in edges:
+            graph.addEdge(node1, node2)
+        return not graph.hasCycle() and all(graph.visited)
+```