public SegmentTreeNode build(int start, int end) {
        // write your code here
        if (start > end)
            return null;
        SegmentTreeNode root = new SegmentTreeNode(start, end);
        if (start != end) {
            root.left = build(start, (start + end) / 2);
            root.right = build((start + end) / 2 + 1, end);
        }
        return root;
    }