Count-Common-Word-Ocuurace.cpp

class Solution {
public:
    int countWords(vector<string>& words1, vector<string>& words2) 
    {
        unordered_map<string, int> m;
        for (auto &w : words1)
            ++m[w];
        for (auto &w : words2)
            if (m[w] < 2)
                --m[w];
        return count_if(begin(m), end(m), [](const auto &p){ return p.second == 0; });
 
    }
};