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:    void dfs(TreeNode* root,TreeNode* &pre,bool &f)    {        if(root==NULL)return;        if(f==false)return;        dfs(root->left,pre,f);        if(pre!=NULL&&pre->val>=root->val)f=false;         pre=root;         dfs(root->right,pre,f);    }    bool isValidBST(TreeNode *root) {        TreeNode* pre=NULL;        bool f = true;        dfs(root,pre,f);        return f;            }};