void doAssignRegion(RegionInfo regionInfo, ServerName serverName) {
    if (!serversToIndex.containsKey(serverName.getAddress())) {
      return;
    }
    int server = serversToIndex.get(serverName.getAddress());
    int region = regionsToIndex.get(regionInfo);
    doAction(new AssignRegionAction(region, server));
  }
Common method for better solution check.
@param colocatedReplicaCountsPerLocation colocatedReplicaCountsPerHost or
                                         colocatedReplicaCountsPerRack
@return 1 for better, -1 for no better, 0 for unknown
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/BalancerClusterState.java