# Description

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)
``````

The original problem is here.

The original code is here.

# My Solution

I solve this problem in C++, as below:

``````/*
*4Sum
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/
#include<iostream>
#include<vector>
#include<map>
#include<stdlib.h>
using namespace std;

class Solution {
public:
vector<vector<int>> result;
vector<vector<int>> fourSum(vector<int>& nums, int target) {
int size = nums.size();
if(size <= 3)
return result;
sort(nums.begin(),nums.end());
for(int i=0;i<size-3;i++)
{
if(i>0 && nums[i]==nums[i-1] )
continue;
for(int j=i+1;j<size-2;j++){
if(j>i+1 && nums[j]==nums[j-1] )
continue;
twoSum(nums, j+1, size-1, nums[i], nums[j], target);
}
}
return result;
}
void twoSum(vector<int> &nums, int start, int end, int num1, int num2, int target){
vector<int> oneSet;

while(start<end){
if(nums[start]+nums[end] + num1 + num2 == target){
oneSet.push_back(num1);
oneSet.push_back(num2);
oneSet.push_back(nums[start]);
oneSet.push_back(nums[end]);
result.push_back(oneSet);
oneSet.clear();
while(start<end && nums[start]==nums[start+1]) start++;
while(start<end && nums[end]==nums[end-1]) end--;
start ++;
end --;
}
else if(nums[start]+nums[end] + num1 + num2 < target)
start ++;
else
end--;
}
return ;
}
};
``````

# Note

For the problem, first sort the array. Then, for a number A in the array, abd next number B, select two numbers(C and D) from the set after A and B, which A+B+C+D=target. When select C and D, we find them from the head and tail of the array.