Convert Sorted Array to Binary Search Tree

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

题目大意：给定排好序的数组，构造二叉搜索树

题目难度：Medium

import java.util.*;
/**
 * Created by gzdaijie on 16/6/8
 * 递归地将数组划分为左右2个部分，分别作为左子树和右子树
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return helper(nums, 0, nums.length - 1);
    }

    public TreeNode helper(int[] nums, int start, int end) {
        if (start > end) return null;

        int mid = (start + end) / 2;
        TreeNode node = new TreeNode(nums[mid]);

        node.left = helper(nums, start, mid - 1);
        node.right = helper(nums, mid + 1, end);
        return node;
    }
}