LeetCode 653. Two Sum IV - Input is a BST--Python解法

题目地址：Two Sum IV - Input is a BST - LeetCode

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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

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这道题目是Two Sum的进阶，最简单的办法是用set，穷举遍历。
Python解法如下：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def findTarget(self, root: TreeNode, k: int) -> bool:
        def find(root, k, s):
            if root == None:
                return False
            if root.val in s:
                flag = 1
                return True
            s.add(k-root.val)
            return find(root.left, k, s) or find(root.right, k, s)
        s = set()
        return find(root, k, s)

时间复杂度为O(n)，空间复杂度为O(n)。

稍微好一点的做法是一层一层遍历，这样可以迭代解决。