Prepare for instant codes

Tool.java

import java.util.stream.*;
import java.util.*;

public class Tool {
  public static Map<Integer, List<Integer>> reverseArrayValue(int[] array) {
    Map<Integer, List<Integer>> result = new HashMap<>();

    for (int i = 0; i < array.length; i++) {
      result.putIfAbsent(array[i], new ArrayList<Integer>());
      result.get(array[i]).add(i);
    }

    return result;
  }

  public static <T> List<T> subtractListBfromA(List<T> a, List<T> b) {
    List<T> bClone = new ArrayList<>(b);
    return a.stream().flatMap(t -> {
      if (bClone.contains(t)) {
        bClone.remove(t);
        return Stream.empty();
      }
      return Stream.of(t);
    }).collect(Collectors.toList());
  }

  public static <T> List<List<T>> splitArray(T[] array, int n) {
    List<List<T>> result = new ArrayList<>(array.length / n);

    List<T> buf = new ArrayList<>(n);
    for (int i = 0; i < array.length; i++) {
      buf.add(array[i]);
      if (i % n == n - 1) {
        result.add(buf);
        buf = new ArrayList<>(n);
      }
    }

    if (buf.size() > 0) {
      result.add(buf);
    }

    return result;
  }

}