【leetcode刷题】T147-两数之和 IV - 输入 BST

案例 1:
输入: 
    5
   / \
  3   6
 / \   \
2   4   7
Target = 9
输出: True

案例 2:
输入: 
    5
   / \
  3   6
 / \   \
2   4   7
Target = 28
输出: False

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def findTarget(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: bool
        """
        self.d = {}
        return self.helper(root, k)

    def helper(self, node, k):
        if not node:
            return False
        if node.val in self.d:
            return True
        self.d[k-node.val] = 1
        return self.helper(node.left, k) or self.helper(node.right, k)