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

如果二叉搜索树中存在2个数之和等于目标值，则返回true

C++（33ms）：

/**
 * 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) {
        unordered_set<int> Set ;
        return dfs(root, k , Set) ;
    }
    
    bool dfs(TreeNode* root, int k , unordered_set<int>& Set) {
        if (root == NULL)
            return false ;
        if (Set.count(k - root->val))
            return true ;
        Set.insert(root->val) ;
        return dfs(root->left,k,Set) ||  dfs(root->right,k,Set) ;
    }
};