653. Two Sum IV - Input is a BST

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
void sol(TreeNode* root,vector<int>& res){
    if(root==NULL)return ;
    sol(root->left,res);
    res.push_back(root->val);
    sol(root->right,res);
}
    bool findTarget(TreeNode* root, int k) {
        vector<int> v;
        sol(root,v);
        for(int i=0;i<v.size();i++){
            int a=k-v[i];
            for(int j=i+1;j<v.size();j++){
                if(v[j]==a)return true;
            }
        }
        return false;
    }
};