原题链接：https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/
将一个有序数组转换成一颗平衡树。虽然平衡树的各种操作的实现我看了几天都没写出来，但是这道题只要知道平衡树是什么就可以解答了：
/**
* Created by clearbug on 2018/2/26.
*/
public class Solution {
static class TreeNode {
int val;
TreeNode left;
TreeNode right;
public TreeNode(int val) {
this.val = val;
}
}
public static void main(String[] args) {
Solution s = new Solution();
TreeNode root = s.sortedArrayToBST(new int[]{1, 3, 5, 7, 9, 11, 13});
System.out.println(root);
}
public TreeNode sortedArrayToBST(int[] nums) {
return sortedArrayToBSTHelper(nums, 0, nums.length - 1);
}
private TreeNode sortedArrayToBSTHelper(int[] nums, int start, int end) {
while (start <= end) {
int middle = (start + end) / 2;
TreeNode node = new TreeNode(nums[middle]);
node.left = sortedArrayToBSTHelper(nums, start, middle - 1);
node.right = sortedArrayToBSTHelper(nums, middle + 1, end);
return node;
}
return null;
}
}