LeetCode108 Convert Sorted Array to Binary Search Tree

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

分析：

因为要构成BBST，所以要从中点开始，左边为左子树，右边为右子树，采用helper函数传递初始结束两个参数，递归求解即可。

代码：

 /**
  * 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(vector<int>& nums, int start, int end) {
         if (start >= end) {
             return nullptr;
         }
         int mid = start + (end - start) / ;
         TreeNode* root = new TreeNode(nums[mid]);
         root -> left = helper(nums, start, mid);
         root -> right = helper(nums, mid + , end);
         return root;
     }
 public:
     TreeNode* sortedArrayToBST(vector<int>& nums) {
         TreeNode* result = helper(nums, , nums.size());
         return result;
     }
 };