comefrom: <awangdev>1203. Sort Items by Groups Respecting Dependencies_1.java
code: private Map<Integer, List<Integer>> buildGroup(int n, int m, int[] group) {
  
          Map<Integer, List<Integer>> map = new HashMap<>();
          for (int i = 0; i < n; i++) {
              int groupId = group[i] == -1 ? m++ : group[i];
              group[i] = groupId;
              map.putIfAbsent(groupId, new ArrayList<>());
              map.get(groupId).add(i);
          }
          return map;
      }
masked: private Map < Integer , List < Integer >> var_0 ( int var_1 , int var_2 , int [ ] var_3 ) {
    Map < Integer , List < Integer >> var_4 = new HashMap < > ( ) ;
    for ( int var_5 = 0 ;
    var_5 < var_1 ;
    var_5 ++ ) {
      int var_6 = var_3 [ var_5 ] == - 1 ? var_2 ++ : var_3 [ var_5 ] ;
      var_3 [ var_5 ] = var_6 ;
      var_4 . putIfAbsent ( var_6 , new ArrayList < > ( ) ) ;
      var_4 . get ( var_6 ) . add ( var_5 ) ;
    }
    return var_4 ;
  }
  