[leetcode] 653. Two Sum IV - Input is a BST

Description

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

题目的意思是：给定一颗二叉树，判断是否存在两个数的和为target。

• 用一个set集合s把来存储每个节点的值，如果再遇见一个target-当前值存在这个集合里面，说明存在两个数，就直接返回正确了。

代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        if(!root) return false;
        unordered_set<int> s;
        return solve(root,k,s);
    }
    bool solve(TreeNode* root, int k,unordered_set<int> &s){
        if(!root){
            return false;
        }
        if(s.count(k-root->val)){
            return true; 
        }
        s.insert(root->val);
        return solve(root->left,k,s)||solve(root->right,k,s);
    }
};

参考文献

​​[LeetCode] Two Sum IV - Input is a BST 两数之和之四 - 输入是二叉搜索树​​