[LintCode] Segment Tree Build

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