leetcode---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>> ans;        sort(nums.begin(), nums.end());        if(nums.size() < 4)            return ans;        if(nums.size() == 4)            if(nums[0] + nums[1] + nums[2] + nums[3] == target)            {                vector<int> tmp;                tmp.push_back(nums[0]);                tmp.push_back(nums[1]);                tmp.push_back(nums[2]);                tmp.push_back(nums[3]);                ans.push_back(tmp);                return ans;            }        int a, b;        int n = nums.size();        for(int i=0; i<n-3; i++)        {            if(i>0 && nums[i] == nums[i-1])                continue;            for(int j=i+1; j<n-2; j++)            {                if(j>i+1 && nums[j] == nums[j-1])                    continue;                int l = j + 1;                int r = n - 1;                while(l < r)                {                    int s = nums[i] + nums[j] + nums[l] + nums[r];                    if(l<r && s < target)                        l++;                    else if(l<r && s > target)                        r--;                    else                    {                        vector<int> tmp;                        tmp.push_back(nums[i]);                        tmp.push_back(nums[j]);                        tmp.push_back(nums[l]);                        tmp.push_back(nums[r]);                        ans.push_back(tmp);                        while(nums[l] == nums[l+1])                            l++;                        while(nums[r] == nums[r-1])                            r--;                        l++;                        r--;                    }                }            }        }        return ans;    }};