ThreeSum.java

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

public class ThreeSum {

    public static List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length - 2; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue; // Skip duplicate elements
            }

            int j = i + 1;
            int k = nums.length - 1;

            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                if (sum == 0) {
                    result.add(Arrays.asList(nums[i], nums[j], nums[k]));

                    // Skip duplicates for j and k
                    while (j < k && nums[j] == nums[j + 1]) {
                        j++;
                    }
                    while (j < k && nums[k] == nums[k - 1]) {
                        k--;
                    }

                    j++;
                    k--;
                } else if (sum > 0) {
                    k--;
                } else {
                    j++;
                }
            }
        }
        return result;
    }

    public static void main(String[] args) {
        System.out.println(threeSum(new int[]{-1, 0, 1, 2, -1, -4}));
        System.out.println(threeSum(new int[]{0, 1, 1}));
        System.out.println(threeSum(new int[]{0, 0, 0}));
        System.out.println(threeSum(new int[]{-2, 0, 1, 1, 2}));
        System.out.println(threeSum(new int[]{1, 2, -2, -1}));
    }
}