[LeetCode] Validate Binary Search Tree

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode *prev;    bool ret;        void DFS(TreeNode *root) {        if (!root) return;                DFS(root->left);                if (prev && prev->val >= root->val) {            ret = false;        }                prev = root;                DFS(root->right);    }        bool isValidBST(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        prev = NULL;        ret = true;        DFS(root);                return ret;    }};

Small Case: 8ms

Large Case: 108ms

Time: O(n)

Space: O(1)