[LeetCode] Convert linked list to BST

/**
 * 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:
    int length(ListNode *head)
    {
        int l=0;
        ListNode *list=head;
        while(list!=NULL)
        {
            list=list->next;
            l++;
        }
        return l;
    }
    
    TreeNode *buildBST(ListNode *&head, int start, int end) //指针引用传递不要忘记 ＊&！！！！！！！！
    {
        if(start > end) { return NULL;}
        
        int med=(start+end)/2;
        TreeNode *leftchild=buildBST(head,start,med-1);
        TreeNode *root=new TreeNode(head->val);
        root->left=leftchild;
        head=head->next;
        TreeNode *rightchild=buildBST(head,med+1,end);
        root->right=rightchild;
        
        return root;
    }
    
    TreeNode *sortedListToBST(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int cnt=length(head);
        return buildBST(head,0,cnt-1);
    }
};