text: class Solution {
      public List<List<Integer>> groupThePeople(int[] a) {
          List<List<Integer>> res = new ArrayList();
          Map<Integer, List<Integer>> data = new HashMap();
          for (int i = 0; i < a.length; i++) {
              List<Integer> tmp = data.computeIfAbsent(a[i], k -> new ArrayList());
              tmp.add(i);
              if (tmp.size() == a[i]) {
                  res.add(new ArrayList(tmp));
                  tmp.clear();
              }
          }
          return res;
      }
  }