98. Validate Binary Search Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return isValidBST(root, NULL, NULL);
    }
    bool isValidBST(TreeNode* root, TreeNode* _min, TreeNode* _max) {
        if (root == NULL)   return true;
        if (_min != NULL && root->val <= _min->val || 
            _max != NULL && root->val >= _max->val) 
            return false;
        return isValidBST(root->left, _min, root) && 
            isValidBST(root->right, root, _max);
    }
};