/**
 * 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) {
        int len = nums.length;
        if( len == 0)
            return null;
        return helper(nums,0,(len-1)/2,len-1);

    }

    public TreeNode helper(int[] nums,int start,int mid,int end){
        
        if( start > end )
            return null;
        TreeNode node = new TreeNode(nums[mid]);

        node.left = helper(nums,start,(mid+start-1)/2,mid-1);

        node.right = helper(nums,mid+1,(end+mid+1)/2,end);

        return node;

    }
}