LeetCode109 Convert Sorted List to Binary Search Tree

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

分析： 

链表不同于数组的是需要先遍历一遍求出长度，然后再找到中点，其余思路与Convert Sorted Array to Binary Search Tree一致。

代码：

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 private:
     TreeNode* helper(ListNode* head, int sz) {
         if (head == nullptr || sz <= ) {
             return nullptr;
         }
         ListNode* runner = head;
         for (int i = ; i < sz / ; ++i) {
             runner = runner -> next;
         }
         TreeNode* root = new TreeNode(runner -> val);
         root -> left = helper(head, sz / );
         root -> right = helper(runner -> next, sz - sz /  - );
         return root;
     }
 public:
     TreeNode* sortedListToBST(ListNode* head) {
         int sz = ;
         ListNode* runner = head;
         while (runner != nullptr) {
             sz++;
             runner = runner -> next;
         }
         TreeNode* result = helper(head, sz);
         return result;
     }
 };