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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if(nums == null) return null;
        return helper(nums,0, nums.length - 1);
    }
    public TreeNode helper(int[] nums, int start, int end){
        if (start > end){
            return null;
        }
        
        int middle = (start + end) / 2;
        TreeNode n = new TreeNode(nums[middle]);
        n.left = helper(nums, start, middle - 1);
        n.right = helper(nums, middle + 1, end);
        return n;
    }
}