18_4Sum.cpp

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        if(nums.empty())
            return res;
        sort(nums.begin(),nums.end());
        int n=nums.size();
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                int target_2=target-nums[j]-nums[i];
                int front = j+1;
                int back = n-1;
                
                while(front<back)
                {
                    int two_sum = nums[front]+nums[back];
                    if(two_sum<target_2)
                        front++;
                    else if(two_sum>target_2)
                        back--;
                    else
                    {
                        vector<int> quadruplet(4,0);
                        quadruplet[0]=nums[i];
                        quadruplet[1]=nums[j];
                        quadruplet[2]=nums[front];
                        quadruplet[3]=nums[back];
                        res.push_back(quadruplet);
                        
                            //processing the duplicates of number 3 
                        while(front<back && nums[front]==quadruplet[2])
                            ++front;
                        
                            //processing the duplicates of number 4
                        while(front<back && nums[back]==quadruplet[3])
                            --back;
                    }
                }
                 ////processing the duplicates of j
                    while(j+1<n && nums[j+1]==nums[j])
                        ++j;
            }
            //processing the duplicates of i
            while(i+1<n && nums[i+1]==nums[i])
                ++i;
        }
        return res;
    }
};