4Sum

similar with 3Sum. O(n³)

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        Arrays.sort(num);
        HashSet<ArrayList<Integer>> hSet = new HashSet<ArrayList<Integer>>();
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        for (int i = 0; i < num.length; i++) {
            for (int j = i + 1; j < num.length; j++) {
                for (int k = j + 1, l = num.length - 1; k < l;) {
                    int sum = num[i] + num[j] + num[k] + num[l];
                    if (sum > target) {
                        l--;
                    }
                    else if (sum < target) {
                        k++;
                    }
                    else if (sum == target) {
                        ArrayList<Integer> found = new ArrayList<Integer>();
                        found.add(num[i]);
                        found.add(num[j]);
                        found.add(num[k]);
                        found.add(num[l]);
                        if (!hSet.contains(found)) {
                            hSet.add(found);
                            result.add(found);
                        }

                    k++;
                    l--;

                    }
                }
            }
        }
    return result;
    }
}