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;    }};