[PAT解题报告] Broken Keyboard

#include <cstdio>
#include <cstring>
#include <string>
#include <cctype>
using namespace std;

bool mark[256];
char s[100],t[100];

int main() {
    scanf("%s%s",s,t);
    
    for (int i = 0, j = 0;s[i];++i) {
        char cs = toupper(s[i]);
        char ct = toupper(t[j]);
        if (s[i] == t[j]) {
            ++j;
        }
        else if (!mark[cs]) {
            mark[cs] = true;
            putchar(cs);
        }
    }
    puts("");
    return 0;
}