653. Two Sum IV - Input is a BST 两个数的和，输入为平衡二叉树

Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 9

Output: True

Example 2:

Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 28

Output: False

题意：给定二进制搜索树和目标数字，如果BST中存在两个元素，使得它们的和等于给定目标，则返回true。

# 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
        """
        if (not root):
            return False
        s = set()
        queue = [root]
        while (queue):
            node = queue.pop(0)
            s.add(node.val)
            if(node.left):
                queue.append(node.left)
            if(node.right):
                queue.append(node.right)

        for num in s:
            if k - num in s and 2 * (k - num) != k:
                return True

        return False

来自为知笔记(Wiz)