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

Solution: 在每段[start, end], mid作为root

    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums == null || nums.length == 0) {
            return null;
        }
        return helper(nums, 0, nums.length - 1);
    }
    private TreeNode helper(int[] nums, int start, int end) {
        if (start > end) {
            return null;
        }
        int mid = (start + end) / 2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = helper(nums, start, mid - 1);
        root.right = helper(nums, mid + 1, end);
        return root;
    }