code: public static String findNodeWithNextLowestSN(List<String> children, String node) {
      List<String> sortedChildren = children.stream()
          .sorted((String o1, String o2) ->
              Long.compare(getSequenceNumber(o1), getSequenceNumber(o2)))
          .collect(Collectors.toList());
  
      int index = sortedChildren.indexOf(node);
      if (index > 0) {
        return sortedChildren.get(index-1);
      } else if (index == 0) {
        return node;
      } else {
        return null;
      }
    }
docs: Finds node with next lowest sequence number than the given node within
  the given list of nodes.
  
  @param children list of children
  @param node the given node to compare sequence number with
  @return the node with next lowest sequence number