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 { // 中序遍历public:    Solution(){        prev = NULL;    }    bool isValidBST(TreeNode* root) {        if (!root)            return true;        if (!isValidBST(root->left))            return false;        if (prev && root->val <= prev->val)            return false;        prev = root;        if (!isValidBST(root->right))            return false;        return true;    }private:    TreeNode* prev;};