[LeetCode] Convert Sorted List to Binary Search Tree

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; *//** * 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 *helper(ListNode *&head, int start, int end)    {        //base case:        if(start > end || head==NULL)            return NULL;                //recursive step: build left subtree, root and right subtree        int median=(start+end)/2;        TreeNode *left=helper(head,start,median-1);        TreeNode *root=new TreeNode(head->val);        root->left=left;        head=head->next;        TreeNode *right=helper(head,median+1,end);        root->right=right;                return root;    }    TreeNode *sortedListToBST(ListNode *head) {        // Note: The Solution object is instantiated only once and is reused by each test case.        int n=0;        ListNode *p=head;        while(p!=NULL){            p=p->next;            n++;        }        return  helper(head,0,n-1);    }};