知识点：字符串处理、哈希表

我的做法

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>
using namespace std;
unordered_map<char, bool> m;
string s, s1, s2;
int main() {
    getline(cin, s1), getline(cin, s2);
    for (int i = 0; s2[i]; i ++ ) m[s2[i]] = true;
    for (int i = 0; s1[i]; i ++ ) if(!m[s1[i]]) s += s1[i];
    cout << s << endl;
    return 0;
}

看了y总后的改进做法

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_set>
using namespace std;
unordered_set<char> m;
string s, s1, s2;
int main() {
    getline(cin, s1), getline(cin, s2);
    for (int i = 0; s2[i]; i ++ ) m.insert(s2[i]);
    for (int i = 0; s1[i]; i ++ ) if(!m.count(s1[i])) s += s1[i];
    cout << s << endl;
    return 0;
}