CODE 114: 4Sum

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 ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {// IMPORTANT: Please reset any member data you declared, as// the same Solution instance will be reused for each test case.if (num.length <= 3) {ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();return results;}ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();Arrays.sort(num);for (int i = 0; i < num.length - 3; i++) {for (int j = i + 3; j < num.length; j++) {int sum = num[i] + num[j];int start = i + 1;int end = j - 1;while (start < end) {if (num[start] + num[end] > target - sum) {while (start < end && num[end - 1] == num[end]) {end--;}end--;} else if (num[start] + num[end] < target - sum) {while (start < end && num[start + 1] == num[start]) {start++;}start++;} else {ArrayList<Integer> result = new ArrayList<Integer>();result.add(num[i]);result.add(num[start]);result.add(num[end]);result.add(num[j]);if (!results.contains(result)) {results.add(result);}start++;end--;}}}while (i < num.length - 3 && num[i + 1] == num[i]) {i++;}}return results;}