Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target. Note: Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d) The solution set must not contain duplicate quadruplets. For example, given array S = {1 0 -1 0 -2 2}, and target = 0. A solution set is: (-1, 0, 0, 1) (-2, -1, 1, 2) (-2, 0, 0, 2) ~~~ public class Solution { public List<List<Integer>> fourSum(int[] num, int target) { Arrays.sort(num); List<List<Integer>> list = new ArrayList<List<Integer>>(); for (int i = 0; i <= num.length - 3; i++) { if (i != 0 && num[i] == num[i-1]) continue; for (int j = i + 1; j <= num.length - 2; j++) { if (j != i + 1 && num[j] == num[j-1]) continue; int left = j + 1, right = num.length - 1; while (left < right) { if (left != j + 1 && num[left] == num[left-1]) { left++; continue; } if (right != num.length - 1 && num[right] == num[right+1]) { right--; continue; } int sum = num[i] + num[j] + num[left] + num[right]; if (sum == target) { List<Integer> item = new ArrayList<Integer>(); item.add(num[i]); item.add(num[j]); item.add(num[left]); item.add(num[right]); list.add(item); left++; right--; } else if (sum < target) { left++; } else { right--; } } // while } // for:j } // for: i return list; } } ~~~