/** * 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:    unordered_set
     
       s;    bool findTarget(TreeNode* root, int k) {        if (root == NULL)   return false;        if (s.find(k - root->val) != s.end()) return true;        s.insert(root->val);        return findTarget(root->left, k) || findTarget(root->right, k);    }};