public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        // Start typing your Java solution below
        // DO NOT write main() function
    Arrays.sort(num);
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
    for (int i = 0; i < num.length - 3;) {
        for (int j = i + 1; j < num.length - 2;) {

            int k = j + 1;
            int l = num.length - 1;

            while (k < l) {
                int sum = num[i] + num[j] + num[k] + num[l];
                if (sum > target) {
                    int prev_l = num[l];
                    while(l > j && num[l] == prev_l) l--;
                }
                else if (sum < target) {
                    int prev_k = num[k];
                    while(k < num.length && num[k] == prev_k) k++;
                }
                else if (sum == target) {
                    ArrayList<Integer> tmp = new ArrayList<Integer>();
                    tmp.add(num[i]);
                    tmp.add(num[j]);
                    tmp.add(num[k]);
                    tmp.add(num[l]);
                    result.add(tmp);

                    while(k < num.length && num[k] == tmp.get(2)) k++;
                    while(l > j && num[l] == tmp.get(3)) l--;
                }
            }
            int prev_j = num[j];
            while(j < num.length - 2 && prev_j == num[j]) j++;
        }
        int prev_i = num[i];
        while(i < num.length - 3 && prev_i == num[i]) i++;
    }
    return result;
    }
}