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/**
 * 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 ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(num.length == 0) return res;
        Arrays.sort(num);
        Set<ArrayList<Integer>> resSet = new HashSet<ArrayList<Integer>>();
        for(int i = 0; i< num.length;i++){
            int j = i+1;
            int k = num.length-1;
            while(j < k){
                int sum_two = num[i]+num[j];
                if(sum_two + num[k] < 0) j++;
                else if(sum_two + num[k] > 0) k--;
                else{
                    //find one triplet
                    ArrayList<Integer> triplet = new ArrayList<Integer>();
                    triplet.add(0, num[i]);
                    triplet.add(1, num[j]);
                    triplet.add(2, num[k]);
                    resSet.add(triplet);
                    j++;
                    k--;
                }
            }
        }
        res.addAll(resSet);
        return res;
    }
}