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) { List<List<Integer>> list = new ArrayList<>(); if (num.length<3) return list; Arrays.sort(num); for (int i=0; i<num.length-2; i++) { if(i!=0 && num[i]==num[i-1]) continue; int left = i+1; int right = num.length-1; while (left<right) { if (left>i+1 && num[left]==num[left-1]) { left++; continue; } if (right<num.length-2 && num[right]==num[right+1]) { right--; continue; } int sum = num[i] + num[left] + num[right]; if (sum==0) { List<Integer> item = new ArrayList<>(); item.add(num[i]); item.add(num[left]); item.add(num[right]); list.add(item); left++; right--; } else if (sum>0) { right--; } else { left++; } } // while } // for return list; } } ~~~