Convert Sorted Array to Binary Search Tree
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
解题思路：
题意为构建有序数组的二分查找树。
比較简单。用递归方法就可以，中间的元素作为根节点，前半部分作为左孩子树，右半部分作为右孩子树。
/*** 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* sortedArrayToBST(vector<int>& nums) {return sortedArrayToBSTHelper(nums, 0, nums.size()-1);}TreeNode* sortedArrayToBSTHelper(vector<int>& nums, int start, int end){if(start>end){return NULL;}int middle = (start+end)/2;TreeNode* root = new TreeNode(nums[middle]);root->left = sortedArrayToBSTHelper(nums, start, middle - 1);root->right = sortedArrayToBSTHelper(nums, middle + 1, end);return root;}
};