4sum leetcode (18)

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>>  vs;        int n = nums.size();        if(n<4)        {            return vs;        }        for(int i=0; i<n-3;)        {             int value = target - nums[i];            int temp = nums[i];            vector<int> v = threeSum(nums,value,i+1,n-1);            int vsize = v.size();            for(int j=0; j<vsize;)            {                vector <int> v4;                v4.push_back(nums[i]);                v4.push_back(v[j++]);                v4.push_back(v[j++]);                v4.push_back(v[j++]);                vs.push_back(v4);            }            while(i<n&&nums[i]==temp)            {                i++;            }        }    }    vector<int> threeSum(vector<int>& nums,int target,int startindex,int endindex)    {        vector<int>  vs;        int n = endindex-startindex+1;        if(n<3)        {            return vs;        }        for(int i=startindex; i<endindex;)        {            int value = target - nums[i];            int temp = nums[i];            vector<int> v = twoSUm(nums,value,i+1,endindex);            int vsize = v.size();            for(int j=0; j<vsize;)            {                vs.push_back(nums[i]);                vs.push_back(v[j++]);                vs.push_back(v[j++]);            }            while(i<n&&nums[i]==temp)            {                i++;            }        }    }    vector<int> twoSUm(vector<int>& nums,int value,int startindex,int endindex)    {        vector<int> v;        while(startindex<endindex)        {            if(nums[startindex] + nums[endindex] == value)            {                v.push_back(nums[startindex]);                v.push_back(nums[endindex]);                while(startindex<endindex && nums[startindex] == nums[startindex+1])                {                    startindex++;                }                startindex++;                while(startindex<endindex && nums[endindex] == nums[endindex-1])                {                    endindex--;                }            }            else if(nums[startindex] + nums[endindex]<value)            {                startindex++;            }            else            {                endindex--;            }        }        return v;    }};