FourSum.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class FourSum {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        // same as 3 sum just another pointer before the computation of threesum

        List<List<Integer>> ans = new ArrayList<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1])
                continue;

            for (int j = i + 1; j < nums.length; j++) {
                if (j > i + 1 && nums[j] == nums[j - 1])
                    continue;

                int k = j + 1, l = nums.length - 1;
                while (k < l) {
                    long sum = (long) nums[i] + nums[j] + nums[k] + nums[l];

                    if (sum == target) {
                        ans.add(Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
                        k++;
                        l--;

                        while (k < l && nums[k] == nums[k - 1])
                            k++;
                        while (k < l && nums[l] == nums[l + 1])
                            l--;

                    } else if (sum > target) {
                        l--;
                    } else {
                        k++;
                    }
                }
            }
        }
        return ans;
    }
}