private void addRegionToMap(Map<ServerName, List<RegionInfo>> assignmentMapForFavoredNodes,
    RegionInfo region, ServerName host) {
    List<RegionInfo> regionsOnServer = assignmentMapForFavoredNodes.get(host);
    if (regionsOnServer == null) {
      regionsOnServer = Lists.newArrayList();
      assignmentMapForFavoredNodes.put(host, regionsOnServer);
    }
    regionsOnServer.add(region);
  }
Return a pair - one with assignments when favored nodes are present and another with regions
without favored nodes.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/FavoredStochasticBalancer.java