4sum implementation in Java from leetcode

Question

The problem statement from leetcode says:

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Fin

Answer 1

Here's a solution that does not use hash. It should be faster, but it still exceeds the time limit.

import java.util.ArrayList;

public class Solution {
    public static void main(String[] args) {

      int[] S = {1, 0, -1, 0, -2, 2};
      int target = 0;
      test(S, target);
    }

    public static void test(int[] num, int target) {
      System.out.print("a:");
      for (int i : num) {
        System.out.print(" " + i);
      }
      System.out.println(": " + target);
      ArrayList> res = fourSum(num, target);
      for (ArrayList list : res) {
        System.out.println(list);
      }
      System.out.println();
    }

    // a+b+c+d = target
    public static ArrayList> fourSum(int[] num, int target) {
        // Start typing your Java solution below
        // DO NOT write main() function

       // Sort
       {
        for (int i = 0; i < num.length; i++) {
            for (int j = i + 1; j < num.length; j++) {
                if (num[j] < num[i]) {
                    int tmp = num[i];
                    num[i] = num[j];
                    num[j] = tmp;
                }
            }
        }
       }
      ArrayList> res = new ArrayList>();
      int i = 0;
      while (i < num.length - 3) {
        int j = i + 1;
        while (j < num.length - 2) {
            int k = j + 1, l = num.length - 1;
            while (k < l) {
                int sum = num[i] + num[j] + num[k] + num[l];
                if (sum > target) {
                    l--;
                    while (k < l && num[l] == num[l+1]) l--;
                } else if (sum < target) {
                    k++;
                    while (k < l && num[k] == num[k-1]) k++;
                } else {
                    ArrayList list =
                        new ArrayList(4);
                    list.add(num[i]); list.add(num[j]);
                    list.add(num[k]); list.add(num[l]);
                    res.add(list);
                    k++;
                    while (k < l && num[k] == num[k-1]) k++;
                    l--;
                    while (k < l && num[l] == num[l+1]) l--;
                }
            }
            j++;
            while (j < num.length && num[j] == num[j-1]) j++;
        }
        i++;
        while (i < num.length && num[i] == num[i-1]) {
            i++;
        }
      }

      return res;
    }
}