1121. Damn Single (25) 格式 map

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

int main(){
    int n, m;
    string a, b;
    cin >> n;
    vector <string> ans, t;
    map<string, string> ma;
    map<string, bool> mb;
    for (int i = 0; i < n; i++) {
        cin >> a >> b;
        ma[a] = b;
        ma[b] = a;
    }

    cin >> m;
    for (int i = 0; i < m; i++) {
        cin >> a;
        t.push_back(a);
        mb[a] = 1;
    }
    for (int i = 0; i < m; i++) {
        if(!mb[ma[t[i]]]) ans.push_back(t[i]);
        //ma[t[i]] t[i]的配偶 mb判断其是否出现在聚会中
    }
    sort(ans.begin(), ans.end());

    cout << ans.size() << endl;
    for (int i = 0; i < ans.size(); i++) {
        if(i != 0) cout << ' ';
        cout << ans[i];
    }
    //呵呵哒
    if (ans.size()) {
        cout << endl;
    }

    return 0;
}