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：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 private:
12     TreeNode* helper(vector<int>& nums, int start, int end) {
13         if (start >= end) {
14             return nullptr;
15         }
16         int mid = start + (end - start) / 2;
17         TreeNode* root = new TreeNode(nums[mid]);
18         root -> left = helper(nums, start, mid);
19         root -> right = helper(nums, mid + 1, end);
20         return root;
21     }
22 public:
23     TreeNode* sortedArrayToBST(vector<int>& nums) {
24         TreeNode* result = helper(nums, 0, nums.size());
25         return result;
26     }
27 };