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.

Note:

• 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.

    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:

    (-1, 0, 1)

    (-1, -1, 2)

public class Solution {

    public List<List<Integer>> threeSum(int[] num) {

        Arrays.sort(num);

        int a,b,c;

        HashSet<List<Integer>> hs=new HashSet();

        for(int i=0;i<=num.length-3;i++){

            a=num[i];

            for(int m=i+1,n=num.length-1; m<n;){

                b=num[m];

                c=num[n];

                if(-a==(b+c)){

                    List<Integer> ls=new ArrayList<Integer>();

                    ls.add(a);

                    ls.add(b);

                    ls.add(c);

                    hs.add(ls);

                    n--;

                    m++;

                }

                else if(-a>(b+c)){

                    m++;

                }

                else{

                    n--;

                }

            }

        }

        List<List<Integer>> result=new ArrayList(hs);

        return result;

}

}

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