code.cpp

/**
 * 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 isValidBSTInternal(TreeNode* root, long sm, long la) {
        if (root == nullptr) {
            return true;
        }

        if (root->val <= sm || root->val >= la) {
            return false;
        }

        if (root->left != nullptr && root->left->val >= root->val) {
            return false;
        }
        if (root->right != nullptr && root->right->val <= root->val) {
            return false;
        }

        return isValidBSTInternal(root->left, sm, root->val) && isValidBSTInternal(root->right, root->val, la);
    }

    bool isValidBST(TreeNode* root) {
        return isValidBSTInternal(root, (long)INT_MIN-1, (long)INT_MAX+1);
    }
};