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, int mn = -1, int mx = -1, bool has_mn = false, bool has_mx = false) {
        if (root == nullptr) {
            return true;
        }

        int curr = root->val;
        if ((has_mn && curr <= mn) || (has_mx && curr >= mx)) {
            return false;
        }

        return isValidBST(root->left, mn, curr, has_mn, true) && isValidBST(root->right, curr, mx, true, has_mx);
    }
};