Find all Triplets with zero sum

Input: a[] = [-5, 3, 2, -3, 1]
Output: [-5, 3, 2] [2, -3, 1]

Input: a[] = [-1, -1, 2, -1, -1, 2, -1, -1, 2]
Output: [-1, -1, 2]

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

public class TripletsWithZeroSum {
    private static List<List<Integer>> findTripletsWithZeroSum(int[] a) {
        List<List<Integer>> triplets = new ArrayList<>();

        Arrays.sort(a);
        for(int i = 0; i < a.length-2; i++) {
            // Skip duplicates
            if(i > 0 && a[i] == a[i-1]) {
                continue;
            }

            // Fix one number
            int firstNum = a[i];

            // Use Two-sum approach to get the other two numbers
            int left = i+1, right = a.length-1;
            while(left < right) {
                int sum = firstNum + a[left] + a[right];
                if(sum == 0) {
                    triplets.add(Arrays.asList(firstNum, a[left], a[right]));
                    left++;
                    right--;

                    // Skip duplicates
                    while(left < right && a[left] == a[left-1]) {
                        left++;
                    }
                    while(left < right && a[right] == a[right+1]) {
                        right--;
                    }
                } else if(sum < 0) {
                    left++;
                } else {
                    right--;
                }
            }
        }

        return triplets;
    }

    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        int n = keyboard.nextInt();
        int[] a = new int[n];
        for(int i = 0; i < n; i++) {
            a[i] = keyboard.nextInt();
        }
        keyboard.close();

        List<List<Integer>> triplets = findTripletsWithZeroSum(a);
        for(List<Integer> triplet: triplets) {
            for(Integer num: triplet) {
                System.out.print(num + " ");
            }
            System.out.println();
        }
    }
}

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

public class TripletsWithZeroSum {
    private static List<List<Integer>> findTripletsWithZeroSum(int[] a) {
        List<List<Integer>> triplets = new ArrayList<>();

        Arrays.sort(a);
        for(int i = 0; i < a.length-2; i++) {
            // Skip duplicates
            if(i > 0 && a[i] == a[i-1]) {
                continue;
            }
            // Fix one number a[i] and find pairs with sum -a[i] starting from index i+1
            addPairsWithTargetSum(a, -a[i], i+1, triplets);
        }

        return triplets;
    }

    private static void addPairsWithTargetSum(int[] a, int targetSum, int left, List<List<Integer>> triplets) {
        int right = a.length-1;

        while(left < right) {
            int sum = a[left] + a[right];
            if(sum == targetSum) {
                triplets.add(Arrays.asList(-targetSum, a[left], a[right]));
                left++;
                right--;

                // Skip duplicates
                while(left < right && a[left] == a[left-1]) {
                    left++;
                }
                while(left < right && a[right] == a[right+1]) {
                    right--;
                }
            } else if(sum < targetSum) {
                left++;
            } else {
                right--;
            }
        }
    }
}