原题出处：https://leetcode.cn/leetbook/read/top-interview-questions-easy/xninbt/

解法一：

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

    public TreeNode sortedArrayToBST(int[] nums,int start,int end) {
        if (start > end) {
            return null;
        }
        int mid = (start+ end) >> 1;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = sortedArrayToBST(nums,start,mid-1);
        root.right = sortedArrayToBST(nums,mid+1,end);
        return root;
    }
}

思路：递归，找到数组元素的中点元素作为节点，中点元素的左边的元素作为左子树，右边元素作为右子树，依次递归即可。