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

ArrayList<ArrayList<Integer>> rl = new ArrayList<ArrayList<Integer>>();

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

HashSet<ArrayList<Integer>> rll = new HashSet<ArrayList<Integer>>();

Arrays.sort(num);

for(int i = 0; i < num.length - 3; i++)

for(int j = i + 1; j < num.length - 2; j++)

for(int m = j + 1, n = num.length - 1; m < n;)

if(num[m] + num[n] == target - num[i] - num[j]){

ArrayList<Integer> tmp = new ArrayList<Integer>();

tmp.addAll(Arrays.asList(num[i], num[j], num[m++], num[n--]));

rll.add(tmp);

}

else if(num[m] + num[n] < target - num[i] - num[j]) m++;

else n--;

rl.addAll(rll);

return rl;

}

}