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

描述：將排序連結串列轉換為平衡的二叉搜尋樹

思路： 後序遍歷，例如：[-10,-3,0,5,9]

                           [-10,-3,0,5,9]                 mid：   3        len : 5
                   [-10,-3]               [5,9]           mid：   2,2      len:  2,2
                         [-10]                   [9]      mid：   1,1      len   1,1
                       []    []                 []    []      
 

程式碼如下：

/**
 * 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 {
public:
    
    TreeNode* PostOrderTraversal(ListNode *root,int len){   //後序遍歷
                 if(len<=0) return NULL;
        
                 int mid = len / 2 + len % 2;
                 int i = mid;
                 ListNode *t = root;
                 while(i>1&&t) {
                      i--;
                      t = t->next;
                 }
                 TreeNode *ret = new TreeNode(t->val);
        
                 ret->left = PostOrderTraversal(root,mid-1);
                 ret->right = PostOrderTraversal(t->next,len-mid);
                 
                 return ret;
    }
    
    TreeNode* sortedListToBST(ListNode* head) {
              int len = 0;
              ListNode * t = head;
              while(t) {
                   len++;
                   t = t->next;
              }
              TreeNode *ret = PostOrderTraversal(head,len);
              return ret;
    }
};