平衡二叉树---将有序数组转换为二叉搜索树

/**
 * 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) {

        
        return helper(nums,0,nums.length - 1);


    }

    public TreeNode helper(int[] nums, int left, int right){

        if(left > right) {

            return null;

        }

        int mid = (left + right) / 2;

        TreeNode root = new TreeNode(nums[mid]);

        root.left = helper(nums, left, mid - 1);

        root.right = helper (nums, mid + 1, right);

        return root; 

    }
}