Java permutation generation

public List<String> permutation(List<String> list, String target, String ans){
  if(target.length() <= 1) {
    list.add(ans + target);
  } else {
    for (int i = 0; i < target.length(); i++) {
      permutation(
        list,
        target.substring(0, i) + target.substring(i + 1),
        ans + target.charAt(i));
    }
  }
  return list;
}

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