【Question】

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) {
        Arrays.sort(num);
        int len=num.length;
        ArrayList<ArrayList<Integer>> result=new ArrayList<ArrayList<Integer>>();
        if(len<3){
            return result;
        }
        int sum=0;
        for(int i=0;i<len-2;i++){
            if(i>0&&num[i]==num[i-1]){
                continue;
            }
            int j=i+1;
            int k=len-1;
            while(j<k){
                if(j>i+1&&num[j]==num[j-1]){
                    j++;
                    continue;
                }
                if(k<len-1&&num[k]==num[k+1]){
                    k--;
                    continue;
                }
                sum=num[i]+num[j]+num[k];
                if(sum>0){
                    k--;
                }else if(sum<0){
                    j++;
                }else{
                    ArrayList<Integer> res =new ArrayList<Integer>();
                    res.add(num[i]);
                    res.add(num[j]);
                    res.add(num[k]);
                    result.add(res);
                    k--;
                }
            }
        }
        return result;
    }
}