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.

3sum.java

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> R = new ArrayList<List<Integer>>();
        int N = nums.length;
        
        for(int i=0; i<N; i++){
            int j=0, k=N-1;
            while(j>=0 && j<i && k>i && k<N) {
                int sum3 = nums[i]+nums[j]+nums[k];
                if(sum3 == 0) {
                    List<Integer> L = new ArrayList<>(3);
                    L.add(nums[j]);
                    L.add(nums[i]);
                    L.add(nums[k]);
                    R.add(L);
                    j++; k--;
                } else if(sum3 > 0) k--;
                else j++;
            }
        }
        
        Set<List<Integer>> S = new HashSet<>();
        List<List<Integer>> T = new ArrayList<List<Integer>>();
        for(List<Integer> e : R){
            if(S.add(e)) T.add(e);
        }
        
        return T;
    }
}