# 18. 4Sum (Medium)

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

Given an array `nums` of n integers and an integer `target`, are there elements a, b, c, and d in `nums` such that a + b + c + d = `target`? Find all unique quadruplets in the array which gives the sum of `target`.

Note:

The solution set must not contain duplicate quadruplets.

Example:

```Given array nums = [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]
]
```

## Solutions

``````class Solution {

// Similar to 3 sum, just add another outer loop

public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> ans = new ArrayList<>();

if (nums.length < 4) {
return ans;
}

Arrays.sort(nums);

for (int i = 0; i <= nums.length - 4; i++) {
if (i != 0 && nums[i] == nums[i - 1]) {
continue; // avoid duplicates
}

for (int j = i + 1; j <= nums.length - 3; j++) {
if (j != i + 1 && nums[j] == nums[j - 1]) {
continue; // avoid duplicates
}

int x = j + 1;
int y = nums.length - 1;

while (x < y) {
if (x != j + 1 && nums[x] == nums[x - 1]) { // avoid duplicates
x++;
continue;
}

if (y != nums.length - 1 && nums[y] == nums[y + 1]) { // avoid duplicates
y--;
continue;
}

int sum = nums[i] + nums[j] + nums[x] + nums[y];

if (sum < target) {
x++;
}

if (sum > target) {
y--;
}

if (sum == target) {
}
}
}
}

return ans;
}
}
``````