Description

Submission

/**
 * 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 {
    stack<TreeNode*> stk;
    long long minVal = 2 * (long long)INT_MIN;
public:
    bool isValidBST(TreeNode* root) {
        if(!root) return true;
        if(!isValidBST(root->left)) return false;
        if(root->val <= minVal) return false;
        minVal = root->val;
        if(!isValidBST(root->right)) return false;
        return true;
    }
};