偷个懒，用个string的库函数

#include <iostream>
#include <cstring>

using namespace std;

int main()
{
    string a, b;
    getline(cin, a);
    getline(cin, b);

    string res;
    for (auto c : a)
    {
        if (b.find(c) == -1)
            res += c;
    }

    cout << res << endl;
    return 0;
}