4Sum.java

/*
    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.
	    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.
    
    Link: https://leetcode.com/problems/4sum/

    Example: 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)

    Solution: None
    
    Source: None
*/

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        HashSet<List<Integer>> hashSet = new HashSet<List<Integer>>();
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        Arrays.sort(nums);
        for (int i = 0; i <= nums.length-4; i++) {
            for (int j = i + 1; j <= nums.length-3; j++) {
                int low = j + 1;
                int high = nums.length - 1;
                while (low < high) {
                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
                    if (sum > target) {
                        high--;
                    } else if (sum < target) {
                        low++;
                    } else if (sum == target) {
                        List<Integer> temp = new ArrayList<Integer>();
                        temp.add(nums[i]);
                        temp.add(nums[j]);
                        temp.add(nums[low]);
                        temp.add(nums[high]);
                        if (!hashSet.contains(temp)) {
                            hashSet.add(temp);
                            result.add(temp);
                        }
                        low++;
                        high--;
                    }
                }
            }
        }
        return result;
    }
}