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.

For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

A solution set is:
(-1, 0, 0, 1)
(-2, -1, 1, 2)
(-2, 0, 0, 2)

public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        // Start typing your Java solution below
        // DO NOT write main() function

        ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
        HashSet<ArrayList<Integer>> resultSet = new HashSet<ArrayList<Integer>>();
        if(num.length<4){
            return output;
        }
        Arrays.sort(num);
        for(int i=0;i<num.length-3;i++){
            for(int j=i+1;j<num.length-2;j++){
                int k = j+1;
                int l = num.length-1;
                while(k<l){
                    int sum = num[i]+num[j]+num[k]+num[l];
                    if(sum==target){
                        ArrayList<Integer> result = new ArrayList<Integer>();
                        result.add(num[i]);
                        result.add(num[j]);
                        result.add(num[k]);
                        result.add(num[l]);
                        if(resultSet.add(result)){
                            output.add(result);
                        }
                        k++;
                        l--;
                    }else if(sum<target){
                        k++;
                    }else{
                        l--;
                    }
                }
            }
        }
        return output;

    }

O(N^3) time and O(n^3) space needed. Waiting for more efficient solutions.

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