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

Arrays.sort(num);

Map <Integer, List<List<Integer>>> map = new HashMap<Integer, List<List<Integer>>>();

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

Set<List<Integer>> result_set = new HashSet<List<Integer>>();

for (int i = 0; i < num.length-1; i++) {

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

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

l.add(i);

l.add(j);

int key = num[i] + num[j];

if (map.containsKey(key) == false) {

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

curr.add(l);

map.put(key, curr);

} else {

List<List<Integer>> curr = map.get(key);

curr.add(l);

map.put(key, curr);

}

}

}

for (Map.Entry<Integer, List<List<Integer>>> entry : map.entrySet()) {

if (map.containsKey(target-entry.getKey())) {

List<List<Integer>> e = map.get(entry.getKey());

List<List<Integer>> t_e = map.get(target-entry.getKey());

for (int i = 0; i < e.size(); i++) {

for (int j = 0; j < t_e.size(); j++) {

if (e.get(i).get(1) < t_e.get(j).get(0) || e.get(i).get(0) > t_e.get(j).get(1)) {

int[] indices = {e.get(i).get(1), e.get(i).get(0), t_e.get(j).get(0), t_e.get(j).get(1)};

Arrays.sort(indices);

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

curr_result.add(num[indices[0]]);

curr_result.add(num[indices[1]]);

curr_result.add(num[indices[2]]);

curr_result.add(num[indices[3]]);

result_set.add(curr_result);

}

}

}

}

}

result.addAll(result_set);

return result;

}

public static void main (String[] args) {

int[] num = {-3, -1, 0, 2, 4, 5};

int target = 0;

Solution sol = new Solution();

System.out.println(sol.fourSum(num, target));

}