ThreeSum.java

package array.part2;

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

public class ThreeSum {
    public static List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> ans = new ArrayList<>();
        int n = nums.length;

        for (int i = 0; i < n - 2 && nums[i] <= 0; ++i) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }

            int j = i + 1, k = n - 1;

            while (j < k) {
                int x = nums[i] + nums[j] + nums[k];
                
                if (x < 0) {
                    ++j;
                } else if (x > 0) {
                    --k;
                } else {
                    ans.add(List.of(nums[i], nums[j++], nums[k--]));
                    while (j < k && nums[j] == nums[j - 1]) {
                        ++j;
                    }
                    while (j < k && nums[k] == nums[k + 1]) {
                        --k;
                    }
                }
            }
        }

        return ans;
    }

    public static void main(String[] args) {
        int[] nums = {-1,0,1,2,-1,-4};

        System.out.println(threeSum(nums));
    }
}

// 3Sum (LeetCode 15)
// https://leetcode.com/problems/3sum/description/