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;
}
    
}