comefrom: <awangdev>261. Graph Valid Tree_3.java
code: private Map<Integer, Set<Integer>> buildGraph(int n, int[][] edges) {
  
          Map<Integer, Set<Integer>> graph = new HashMap<>();
          for (int i = 0; i < n; i++) graph.putIfAbsent(i, new HashSet<>());
          
          for (int[] edge: edges) { 
              graph.get(edge[0]).add(edge[1]);
              graph.get(edge[1]).add(edge[0]);
          }
          return graph;
      }
masked: private Map < Integer , Set < Integer >> var_0 ( int var_1 , int [ ] [ ] var_2 ) {
    Map < Integer , Set < Integer >> var_3 = new HashMap < > ( ) ;
    for ( int var_4 = 0 ;
    var_4 < var_1 ;
    var_4 ++ ) var_3 . putIfAbsent ( var_4 , new HashSet < > ( ) ) ;
    for ( int [ ] var_5 : var_2 ) {
      var_3 . get ( var_5 [ 0 ] ) . add ( var_5 [ 1 ] ) ;
      var_3 . get ( var_5 [ 1 ] ) . add ( var_5 [ 0 ] ) ;
    }
    return var_3 ;
  }
  