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

 

class Solution {

    public TreeNode sortedArrayToBST(int[] nums) {

        if (nums.length == 0) return null;

        TreeNode head = helper(nums, 0, nums.length - 1);

        return head;

    }

    public TreeNode helper(int[] num, int low, int high) {

        if (low > high)

　　　　return null;

        int mid = low + (high - low) / 2;

        TreeNode node = new TreeNode(num[mid]);

        node.left = helper(num, low, mid - 1);

        node.right = helper(num, mid + 1, high);

        return node;

    }

}