- class Solution {
- public:
- intfourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
- intn = A.size();if(!n)return 0;
- intret =0;
- unordered_map<int, unsigned> hm1, hm2;
- for(inti =0; i < n; i ++)
- for(intj =0; j < n; j ++)
- hm1[A[i] + B[j]] ++, hm2[C[i] + D[j]] ++;
- for(auto &kv: hm1)
- if(hm2.find(-kv.first) != hm2.end())
- ret += kv.second * hm2[-kv.first];
- return ret;
- }
- };
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