public class Solution {

public List<List<Integer>> fourSum(int[] num, int target) {

List<List<Integer>> ret = new ArrayList<>();

if (num == null || num.length < 4) return ret;

Arrays.sort(num);

for (int v=0; v < num.length-2; v++){

for(int i=v+1; i< num.length-1; i++){

int left = i+1;

int right = num.length -1;

while (left < right){

int sum = num[v]+ num[i] + num[left] + num[right];

if (sum == target){

List<Integer> result = Arrays.asList(num[v], num[i], num[left], num[right]);

if (!ret.contains(result)) ret.add(result);

left++;

right--;

while(left<right&&num[left]==num[left-1]){

left++;

}

while(left<right&&num[right]==num[right+1]){

right--;

}

}

else if(sum < target){

left++;

}

else{

right--;

}

}

}

}

return ret;

}

}