对二叉树进行dfs

 1 # Definition for a binary tree node.
 2 # class TreeNode(object):
 3 #     def __init__(self, x):
 4 #         self.val = x
 5 #         self.left = None
 6 #         self.right = None
 7 
 8 class Solution(object):
 9     def findTarget(self, root, k):
10         """
11         :type root: TreeNode
12         :type k: int
13         :rtype: bool
14         """
15         if(root==None):
16             return False
17 
18         candidates=set()
19         stack=[root]
20         while len(stack)!=0:
21             cur=stack.pop()
22 
23             if k-cur.val in candidates:
24                 return True
25             else:
26                 candidates.add(cur.val)
27             
28             if(cur.left!=None):
29                 stack.append(cur.left)
30 
31             if(cur.right!=None):
32                 stack.append(cur.right)
33         
34         return False