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/**
 * 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) {}
 * };
 */
 
#include <bits/stdc++.h> 
using namespace std;
class Solution {
public:
    vector<int> help(TreeNode* root ){
        vector<int> res;
        if(root == nullptr) return res;

        if(root->right){
            res.push_back(root->right->val);
           vector<int> rightT = help(root->right);
           res.insert(res.end(), rightT.begin(), rightT.end()); 
        }  
        if(root->left){
            res.push_back(root->left->val);
            vector<int> leftT = help(root->left);
            res.insert(res.end(),leftT.begin(),leftT.end());
        }     
        return res;
    }

    bool isValidBST(TreeNode* root) {
        if(root == nullptr){
            return true;
        }
        if(root->right){
            vector <int> h = help(root->right);

            bool x = true;
            for(auto&element : h){
                if(element <= root->val) {x = false;break;}
            }
            if(!x || root->val >= root->right->val || !isValidBST(root->right)) return false;
        }

        if(root->left){
            vector<int> subL = help(root->left);
            bool x = true;
            for(auto& element: subL){
                if(element >= root->val){
                    x = false;
                    break;
                }
            }
            if(!x || root->val <= root->left->val || !isValidBST(root->left)) return false;

            // return x && root->val > root->left->val && isValidBST(root->left);
        }
        return true;

    }
};