Validate Binary Search Tree

<-M 98> Validate Binary Search Tree

/**
 * 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) {
        return isValidBST(root, LLONG_MIN, LLONG_MAX);
    }
private:
    bool isValidBST(TreeNode* root, long long int lower, long long int upper) {
        if(root == nullptr)
            return true;
        return root->val > lower && root->val < upper
            && isValidBST(root->left, lower, root->val)
            && isValidBST(root->right, root->val, upper);
    }
};