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? Find all unique quadruplets in the array which gives the sum of target.

Note:

Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.

About this four sum problem, I have 2 questions:

1. Where I went wrong? The compiled code cannot pass all the tests, but I thought the code should be right since it is only using brute force to solve the problem.
2. Is there any efficient way to solve the four sum problem instead of this O(n^4) run-time algorithm?

---

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;

public class FourSumSolution{
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target){
        ArrayList<ArrayList<Integer>> aList = new ArrayList<ArrayList<Integer>>();
        int N = num.length;

        for(int i=0; i<N; i++)
            for(int j=i+1; j<N; j++)
                for(int k=j+1; k<N; k++)
                    for(int l=k+1; l<N; l++){
                        int sum = num[i] + num[j] + num[k] + num[l];                        
                        if(sum == target){
                            ArrayList<Integer> tempArray = new ArrayList<Integer>();
                            Collections.addAll(tempArray, num[i], num[j], num[k], num[l]);
                            aList.add(tempArray);
                        }
                    }
        return aList;   
    }
}

Answer 1

Here is an O(n^3) solution

import java.util.Arrays;
import java.util.ArrayList;
import java.util.HashSet;
public class Solution {
public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
    // Start typing your Java solution below
    // DO NOT write main() function

    Arrays.sort(num);
    HashSet<ArrayList<Integer>> hSet = new HashSet<ArrayList<Integer>>();
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
    for (int i = 0; i < num.length; i++) {
        for (int j = i + 1; j < num.length; j++) {
            for (int k = j + 1, l = num.length - 1; k < l;) {
                int sum = num[i] + num[j] + num[k] + num[l];
                if (sum > target) {
                    l--;
                }
                else if (sum < target) {
                    k++;
                }
                else if (sum == target) {
                    ArrayList<Integer> found = new ArrayList<Integer>();
                    found.add(num[i]);
                    found.add(num[j]);
                    found.add(num[k]);
                    found.add(num[l]);
                    if (!hSet.contains(found)) {
                        hSet.add(found);
                        result.add(found);
                    }

                    k++;
                    l--;

                }
            }
        }
    }
    return result;
}

}