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

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

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   7Target = 9Output: True

Example 2:

Input: 5/ \3   6/ \   \
2   4   7Target = 28Output: False

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 Falseif root.val in s:flag = 1return Trues.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。

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