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: 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]
]

answer:

class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
sort(nums.begin(),nums.end());
int sum = 0;
int first, second,third,forth;
//int preFirst, preSecond,preThird,preForth;
vector<vector<int>> result;
for(first = 0; first < nums.size(); first ++){
if(first > 0 ){
while(nums[first] == nums[first - 1]) {
first ++;
}
}
//preFirst = nums[first];
for(second = first + 1; second < nums.size(); second ++){
if(second - first > 1 ){
while(nums[second] == nums[second - 1]) {
second ++;
}
}
//preFirst = nums[first];
third = second + 1;
forth = nums.size() - 1;
while(third < forth){
sum = nums[first] + nums[second] + nums[third] + nums[forth];
if(sum < target) third ++;
else if(sum > target) forth --;
else{
vector<int> temp;
temp.push_back(nums[first]);
temp.push_back(nums[second]);
temp.push_back(nums[third]);
temp.push_back(nums[forth]);
result.push_back(temp);
third ++;
while(nums[third] == nums[third - 1]) third ++;
forth --;
while(nums[forth] == nums[forth + 1]) forth --;
}
}
}
}
return result;
}
};