4 Sum

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]
]

Solution:

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> ans = new ArrayList<>();
        Arrays.sort(nums);
        for(int i=0; i<nums.length-3; i++){
            if(i!=0 && nums[i]==nums[i-1]) continue;
            for(int j=i+1; j<nums.length-2; j++){
                if(j!=i+1 && nums[j]==nums[j-1]) continue;
                int l=j+1, r=nums.length-1;
                int two = target-nums[i]-nums[j];
                while(l<r){
                    int sum = nums[l]+nums[r];
                    if(sum<two){
                        l++;
                        while(l<r && nums[l]==nums[l-1]) l++;
                    } else if(sum>two){
                        r--;
                        while(l<r && nums[r]==nums[r+1]) r--;
                    } else {
                        List<Integer> tmp = new ArrayList<>();
                        tmp.add(nums[i]);
                        tmp.add(nums[j]);
                        tmp.add(nums[l]);
                        tmp.add(nums[r]);
                        ans.add(tmp);
                        l++;
                        r--;
                        while(l<r && nums[l]==nums[l-1]) l++;
                        while(l<r && nums[r]==nums[r+1]) r--;
                    }
                }
            }
        }
        return ans;
    }
}