/**
 * 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 isValidBSTRec(TreeNode* root, int* min, int* max) {
        if (root->left && root->right) {
            int leftMin, leftMax, rightMin, rightMax;
            if (!isValidBSTRec(root->left,  &leftMin,  &leftMax) ||
                !isValidBSTRec(root->right, &rightMin, &rightMax)) {
                return false;
            }
            *min = std::min(std::min(leftMin, rightMin), root->val);
            *max = std::max(std::max(leftMax, rightMax), root->val);
            return (leftMax < root->val) && (root->val < rightMin);
        }
        else if (root->left) {
            int leftMin, leftMax;
            if (!isValidBSTRec(root->left, &leftMin, &leftMax)) {
                return false;
            }
            *min = std::min(leftMin, root->val);
            *max = std::max(leftMax, root->val);
            return (leftMax < root->val);
        }
        else if (root->right) {
            int rightMin, rightMax;
            if (!isValidBSTRec(root->right, &rightMin, &rightMax)) {
                return false;
            }
            *min = std::min(rightMin, root->val);
            *max = std::max(rightMax, root->val);
            return (root->val < rightMin);
        }
        else {
            *min = *max = root->val;
            return true;
        }
    }

    bool isValidBST(TreeNode* root) {
        if (!root) return true;
        int minVal, maxVal;
        return isValidBSTRec(root, &minVal, &maxVal);
    }
};