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 class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
    	List<List<Integer>> res = new ArrayList<List<Integer>>();
    	Arrays.sort(nums);
    	HashSet<List<Integer>> hashSet = new HashSet<List<Integer>>();
    	for (int i = 0; i < nums.length-3; i++) {
			int a = nums[i];
			for (int j = i+1; j < nums.length-2; j++) {
				int start = j+1;
				int end = nums.length-1;
				while (start < end) {
					int sum = nums[i] + nums[j] + nums[start] + nums[end];
					if (target < sum) {
						end--;
					} else if (target > sum) {
						start++;
					}  else {
						ArrayList<Integer> arrayList = new ArrayList<>();
						arrayList.add(nums[i]);
						arrayList.add(nums[j]);
						arrayList.add(nums[start]);
						arrayList.add(nums[end]);
						hashSet.add(arrayList);
						start++;
						end--;
					}
				}
			}
		}
    	res.addAll(hashSet);
    	return res;
    }
}