leetcode——18——4Sum

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)

class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> out;
set<vector<int>> res;
if (nums.size() < 4)
return out;
sort(nums.begin(), nums.end());
for (int i=0; i<nums.size()-3; i++)
{

for (int j =i+1;j<nums.size()-2;j++)
{
int begin = j+1;
int end = nums.size()-1;
while(begin < end){
int sum = nums[i] + nums[j] + nums[begin] + nums[end];
if (sum== target)
{
vector<int> cur(4);
cur[0] = nums[i];
cur[1] = nums[j];
cur[2] = nums[begin];
cur[3] = nums[end];
res.insert(cur);
begin++;
end--;
}
else if(sum < target)
{
begin++;
}else{
end--;
}
}

}
}
set<vector<int>>::iterator it = res.begin();
for(; it != res.end(); it++)
out.push_back(*it);

return out;
}
};