Three Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero. Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c) The solution set must not contain duplicate triplets.

Example:

For example, given array S = {-1 0 1 2 -1 -4}, A solution set is:

(-1, 0, 1)

(-1, -1, 2)

Solution:

Extension of Two Sum. Run two sum inside a for loop.

Code:

public class Solution {
    /**
     * @param numbers : Give an array numbers of n integer
     * @return : Find all unique triplets in the array which gives the sum of zero.
     */
    public ArrayList<ArrayList<Integer>> threeSum(int[] numbers) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (numbers == null || numbers.length == 0) {
            return res;
        }
        Arrays.sort(numbers);
        HashSet<ArrayList<Integer>> hset = new HashSet<>();
        for (int i = 0; i < numbers.length; i++) {
            int left = i + 1;
            int right = numbers.length - 1;
            while (left < right) {
                int num = numbers[i] + numbers[left] + numbers[right];
                if (num == 0) {
                    ArrayList<Integer> temp = new ArrayList<>();
                    temp.add(numbers[i]);
                    temp.add(numbers[left]);
                    temp.add(numbers[right]);
                    Collections.sort(temp);
                    hset.add(temp);
                    left++;
                    right--;
                } else if (num > 0) {
                    right--;
                } else {
                    left++;
                }
            }
        }
        for (ArrayList<Integer> temp : hset) {
            res.add(temp);
        }
        return res;
    }
}