Convert Sorted List to Binary Search Tree

来源：Leetcode

原帖：https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/

题目：
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

代码：

 /**  
  * 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 getLength(ListNode *head) {  
     int length = 0;  
     while (head) {  
       length++;  
       head = head->next;  
     }  
     return length;  
   }  
   
   TreeNode *sortedListToBSTHelper(ListNode* &head, int length) {  
     if (length == 0) return NULL;  
     TreeNode* left = sortedListToBSTHelper(head, length / 2);  
     TreeNode* root = new TreeNode(head->val);  
     head = head->next; // move head pointer!  
     TreeNode* right = sortedListToBSTHelper(head, length - length / 2 - 1);  
     root->left = left;  
     root->right = right;    
     return root;  
   }  
   
   TreeNode *sortedListToBST(ListNode *head) {  
     return sortedListToBSTHelper(head, getLength(head));  
   }  
 };