Distinct values

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

public class MyClass{

    public static void main(String[] args){
        var arr = new ArrayList<Integer>(List.of(1, 1, 2, 4, 4, 5));
        System.out.println(method1(arr));
        System.out.println(method2(arr));
        System.out.println(method3(arr));
    }

    // Fast. O(n) time complexity
    private static List<Integer> method1(List<Integer> arr) {
        return arr.stream().distinct().collect(Collectors.toList());
    }

    // Fast. O(n) time complexity
    private static List<Integer> method2(List<Integer> arr) {
        var taken = new HashSet<Integer>();
        var result = new ArrayList<Integer>();

        for (int i = 0; i < arr.size(); i++) {
            var value = arr.get(i);
            if (!taken.contains(value)) {
                taken.add(value);
                result.add(value);
            }
        }

        return result;
    }

    // Slow. O(n^2) time complexity
    private static List<Integer> method3(List<Integer> arr) {
        var result = new ArrayList<Integer>();

        for (int i = 0; i < arr.size(); i++) {
            var value = arr.get(i);
            var exists = false;

            for (int j = 0; j < result.size(); j++) {
                if (Objects.equals(result.get(j), value)) {
                    exists = true;
                    break;
                }
            }

            if (!exists) result.add(value);
        }

        return result;
    }
}