int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {

vector<vector<int>> result;

sort(A.begin(),A.end());

sort(B.begin(),B.end());

unordered_map<int,vector<pair<int,int>>> cache;

for (size_t i = 0; i < A.size(); ++i) {

for (size_t j = 0; j < B.size(); ++j) {

cache[A[i]+B[j]].push_back(make_pair<int,int>(i,j));

}

}

for (size_t k = 0; k < C.size(); ++k) {

for (size_t l = 0; l < D.size(); ++l) {

const int key = 0-C[k]-D[l];

if(cache.find(key) == cache.end()) continue;

const auto &vec = cache[key];

for (int m = 0; m < vec.size(); ++m) {

result.push_back({A[vec[key].first],B[vec[key].second],C[k],D[l]});

}

}

}

sort(result.begin(),result.end());

result.erase(unique(result.begin(),result.end()),result.end());

return result.size();

}

int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {

unordered_map<int, int> abSum;

for(auto a : A) {

for(auto b : B) {

++abSum[a+b];

}

}

int count = 0;

for(auto c : C) {

for(auto d : D) {

auto it = abSum.find(0 - c - d);

if(it != abSum.end()) {

count += it->second;

}

}

}

return count;

}