108. 将有序数组转换为二叉搜索树

给定有序数组: [-10,-3,0,5,9],

一个可能的答案是：[0,-3,9,-10,null,5]，它可以表示下面这个高度平衡二叉搜索树：

      0
     / \
   -3   9
   /   /
 -10  5

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums == null || nums.length == 0) {
            return null;
        }
        return buildTree(nums, 0, nums.length - 1);
    }

    private TreeNode buildTree(int[] nums, int left, int right) {
        if (left > right) {
            return null;
        }
        int mid = left + (right - left) / 2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = buildTree(nums, left, mid - 1);
        root.right = buildTree(nums, mid + 1, right);
        return root;
    }
}