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: 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 class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        HashSet<ArrayList<Integer>> hashset = new HashSet<ArrayList<Integer>>();
        List<List<Integer>> rec = new LinkedList<>();//这里还不知道为什么可以这样
        if(nums.length<4||nums==null)
          return rec;
        Arrays.sort(nums);
        
        for(int i=0;i<nums.length-3;i++)
          for(int j=i+1;j<nums.length-2;j++)
            {
                int k = j+1;
                int l = nums.length-1;
                while(k<l)
                 {
                     int sum = nums[i]+nums[j]+nums[k]+nums[l];
                     if(sum>target)
                        l--;
                     else if(sum<target)
                        k++;
                     else
                        {
                            ArrayList<Integer> result = new ArrayList<Integer>();
                            result.add(nums[i]);
                            result.add(nums[j]);
                            result.add(nums[k]);
                            result.add(nums[l]);
                            if(!hashset.contains(result))
                                hashset.add(result);
                            k++;
                            l--;
                        }
                 }
            }
            
        
        rec.addAll(hashset);
        return rec;
    }
}