#include <bits/stdc++.h>

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 != nullptr) {
            return valid(root, INT_MIN, INT_MAX, false, false);
        }
        else {
            return true;
        }
    }
    bool valid(TreeNode* t, int min, int max, bool wasMin, bool wasMax) {
        if ((wasMin && t->val <= min) || (wasMax && t->val >= max)) {
            return false;
        }
        if (t->left != nullptr) {
            bool l = valid(t->left, min, t->val, wasMin, true);
            if (t->right != nullptr) {
                return l && valid(t->right, t->val, max, true, wasMax);
            }
            else {
                return l;
            }
        }
        else {
            if (t->right != nullptr) {
                return valid(t->right, t->val, max, true, wasMax);
            }
            else {
                return true;
            }
        }
    }
};