如何从 ArrayList 的 ArrayList 中删除重复的 ArrayList

Question

Problem: Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

My Code:

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(ArrayList<Integer> A) {
        ArrayList<ArrayList<Integer>> C = new  ArrayList<ArrayList<Integer>>();
        int n = A.size();
        for(int i =0; i<n-2; i++){
            for(int j=i+1; j<n-1; j++){
                for(int k=j+1; k< n; k++){
                    int sum = A.get(i)+A.get(j)+A.get(k);
                    if(sum == 0){
                        ArrayList<Integer> temp = new ArrayList<Integer>();
                        temp.add(A.get(i));
                        temp.add(A.get(j));
                        temp.add(A.get(k));
                        C.add(temp);
                    }
                }
            }

        }
      return C;
    }
}

So C may contains repeated Arraylist, and my aim is to remove repeated Arraylist from C

Example: C = [-5 1 4 ] [-5 1 4 ] [-5 1 4 ] [-5 4 1 ] [-4 0 4 ] [-4 0 4 ]
My aim to get = [-5 1 4 ] [-5 4 1 ] [-4 0 4 ]

Please suggest me some ways to do some operation on C so that I can do it.

Answer 1

The equals method of AbstractList (which ArrayList extends) is defined so that two lists are equal if they contain the same elements in the same order. The simplest way then would be to get the distinct lists from a stream:

List<List<Integer>> list = new ArrayList<>();
list.add(Arrays.asList(-5, 1, 4));
list.add(Arrays.asList(-5, 1, 4));
list.add(Arrays.asList(-5, 4, 1));
list.add(Arrays.asList(-4, 0, 4));
list.add(Arrays.asList(-4, 0, 4));
list.add(Arrays.asList(-4, 0, 4));

List<List<Integer>> distinctLists = list.stream().distinct().collect(Collectors.toList());

System.out.println(distinctLists); // prints [[-5, 1, 4], [-5, 4, 1], [-4, 0, 4]]

Answer 2

As given in previous answer, create a list of list
List<List<Integer>> listToBeUpdated = new ArrayList<>();
list.add(Arrays.asList(-5, 1, 4));
list.add(Arrays.asList(-5, 1, 4));
list.add(Arrays.asList(-5, 4, 1));
list.add(Arrays.asList(-4, 0, 4));
list.add(Arrays.asList(-4, 0, 4));
list.add(Arrays.asList(-4, 0, 4)); 

Then, You can achieve this using Set(as set do not allow duplicates). in Java 8:

    listToBeUpdated.stream().flatMap(List::stream).collect(Collectors.toSet())

Or you can simply:

    public List<Integer> removeDuplicatesFromList(List<List<Integer>> listToBeUpdated) {
        Set<Integer> uniqueList = new HashSet<Integer>();    //Use TreeSet<Integer>, for sorting order or LinkedHashSet<Integer> for insertion order
        for(List<Integer> anInt: list) {
            uniques.addAll(anInt);
        }
        return new ArrayList<>(uniqueList);   // If you want to return list
    }

Hope it helps !