[Leetcode] 108. Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

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