two_sum.cpp

/**
 * 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) {
        set<int> s; 
        return  dfs(root,k,s); 
        
    }
    bool dfs(TreeNode* root,int k,set<int>& s){
        if(root == nullptr)
            return false; 
        if(s.find(root->val ) != s.end())
            return true; 
        s.insert(k-root->val ); 
        return dfs(root->left ,k,s) || dfs(root->right,k,s); 
    }
};