LeetCode.653 Two Sum IV - Input is a BST

题目：

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

分析：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean findTarget(TreeNode root, int k) {
//给定规定二叉树，以及给定目标值，返回是否二叉树中是否存在两个节点的和等于目标值
//思路：利用set存储已经遍历的节点，递归查找
return backtrace(root,new HashSet<>(),k);
}
public boolean backtrace(TreeNode root,Set<Integer> set,int k){
if(root==null) return false;
if(set.contains(k-root.val)){
//说明存在剩余值
return true;
}
set.add(root.val);
return backtrace(root.left,set,k)||backtrace(root.right,set,k);
}
}