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
     
      
       > fourSum(int[] nums, int target) {        List
       
        
         > resList = new ArrayList
         
          
           >(); if(nums==null || nums.length<4){ return resList; } Arrays.sort(nums); int len=nums.length; if(nums[0]>0){ return resList; } else if(nums[len-1]<0){ return resList; } else{ for(int i=0; i
           
            0 && nums[i]==nums[i-1])){ int first=nums[i]; for(int j=i+1; j
            
             i+1 && nums[j] == nums[j-1]) ){ int second=nums[j]; findTwoNumbers(nums,target,first,second, j+1, resList); } } } } } return resList; } public void findTwoNumbers(int[] nums, int target, int first, int second, int start, List
             
              
               > resList){ int left=start; int right=nums.length-1; while(left
               
                 list=new ArrayList
                
                 (); list.add(first); list.add(second); list.add(nums[left]); list.add(nums[right]); resList.add(list); //System.out.println(resList.toString()); left++; right--; } else if(nums[left]+nums[right]>(target-first-second)){ right--; } else{ left++; } } } }}