# 4 Sum

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)
``````

## Solution

Similar to 3Sum, 2Sum. 但是又很多细节需要注意

## Code

``````public class Solution {
public List<List<Integer>> fourSum(int[] num, int target) {
int N = num.length;
List<List<Integer>> res = new ArrayList<List<Integer>>();
if (N < 4) return res;
Arrays.sort(num);
for (int i = 0; i < N; ++i)
{
if (i > 0 && num[i] == num[i-1]) continue; // avoid duplicates
for (int j = i+1; j < N; ++j)
{
if (j > i+1 && num[j] == num[j-1]) continue; // avoid duplicates
int twosum = target - num[i] - num[j];
int l = j + 1, r = N - 1;
while (l < r)
{
int sum = num[l] + num[r];
if (sum == twosum) {
ArrayList<Integer> tmp = new ArrayList<Integer>();