# 4Sum

## Problem

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.

## 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)

## 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.

## Code - Java

public class Solution {
/**
* @param numbers : Give an array numbersbers of n integer
* @param target  : you need to find four elements that's sum of target
* @return : Find all unique quadruplets in the array which gives the sum of
* zero.
*/
public ArrayList<ArrayList<Integer>> fourSum(int[] numbers, int target) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();

Arrays.sort(numbers);

for (int i = 0; i < numbers.length - 3; i++) {
for (int j = i + 1; j < numbers.length - 2; j++) {
int left = j + 1;
int right = numbers.length - 1;

while (left < right) {
int sum = numbers[i] + numbers[j] + numbers[left] + numbers[right];
if (sum == target) {
ArrayList<Integer> tuple = new ArrayList<>();
if (!result.contains(tuple)) {
}
left++;
right--;
} else if (sum < target) {
left++;
} else {
right--;
}
}
}
}
return result;
}
}