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]
]

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class Solution{
public:
    vector<vector<int>> fourSum(vector<int> & nums,int target){
        vector<vector<int>> res;
        if(nums.size()<4)
            return res;

        sort(nums.begin(),nums.end());
        unordered_map<int,vector<pair<int,int>>> cache;
        for(size_t i =0 ;i< nums.size();i++){
            for (size_t j = i+ 1;j<nums.size();j++){
                cache[nums[i]+nums[j]].push_back(pair<int,int>{i,j});
            }
        }

        for(size_t i=0;i<nums.size();i++){
            for (size_t j = i+1;j<nums.size();j++){
                int key = target - nums[i] - nums[j];
                if(cache.find(key) == cache.end()){
                    continue;
                }

                vector<pair<int,int>> vec = cache[key];
                for(size_t k = 0;k < vec.size();k++){
                    if(vec[k].first <= j)
                        continue;
                    res.push_back({nums[vec[k].first],nums[vec[k].second],nums[i],nums[j]});
                }
            }
        }
        sort(res.begin(),res.end());
         res.erase(unique(res.begin(),res.end()),res.end());
        return res;
    }
};