题目描述：

LeetCode 653. Two Sum IV - Input is a BST

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

题目大意：

给定二叉搜索树BST与一个目标数target，判断BST中是否存在两个数之和等于target。

解题思路：

解法I 递归遍历BST + 利用BST性质进行检索

时间复杂度O(M * H), M是树中节点个数，H是树的深度

Python代码：

# 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.root = root
        self.k = k
        return self.findNumber(root)
    def findNumber(self, root):
        if not root: return False
        node = self.root
        n = self.k - root.val
        if n != root.val:
            while node:
                if node.val == n: return True
                if n > node.val: node = node.right
                else: node = node.left
        return self.findNumber(root.left) or self.findNumber(root.right)

解法II 递归遍历BST + Two Sum

时间复杂度O(M)，空间复杂度O(M)；M是树中节点个数

Python代码：

# 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.dset = set()
        self.traverse(root)
        for n in self.dset:
            if k - n != n and k - n in self.dset:
                return True
        return False
    def traverse(self, root):
        if not root: return
        self.dset.add(root.val)
        self.traverse(root.left)
        self.traverse(root.right)

 

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