class Solution {
public:
int countWords(vector<string>& words1, vector<string>& words2) {
unordered_map<string, int> h1, h2;
for (auto c: words1) h1[c] ++ ;
for (auto c: words2) h2[c] ++ ;
int res = 0;
for (auto [k, v]: h1)
if (v == 1 && h2[k] == 1) res ++ ;
return res;
}
};