leetcode Validate Binary Search Tree

代码

class Solution {public: bool isValidBST(TreeNode *root) {     pre = NULL;     return isValidBSTHelper(root);    }   bool isValidBSTHelper(TreeNode *root) {    if(root)    {        if (!isValidBSTHelper(root->left))          return false;        // Allows only distinct valued nodes        if (pre!= NULL && root->val <= pre->val)          return false;        pre = root;        return isValidBSTHelper(root->right);    }    return true;      }  private: TreeNode *pre; };