/**
 * 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:
    bool isValidBST(TreeNode* root) {
        if(!root)return true;
        return dfs(root)[0];
    }

    vector<int> dfs(TreeNode* root)
    {
        vector<int> res({1, root->val, root->val});
        if(root->left)
        {
            auto t=dfs(root->left);
            if(!t[0]||t[2]>=root->val)res[0]=0;
            res[1]=min(res[1], t[1]);
            res[2]=max(res[2], t[2]);
        }
        if(root->right)
        {   
            auto t=dfs(root->right);
            if(!t[0]||t[2]<=root->val)res[0]=0;
            res[1]=min(res[1], t[1]);
            res[2]=max(res[2], t[2]);
        }
        return res;
    }
};