PAT (Advanced Level) 1061. Dating (20)

#include<stdio.h>
#include<string.h>

char s1[70],s2[70],s3[70],s4[70];
char f[7][5]={"MON ", "TUE ","WED ","THU ","FRI ","SAT ","SUN "};
int a,b,c,flag;

int main()
{
    scanf("%s%s%s%s",s1,s2,s3,s4); flag=0;
    for(int i=0;s1[i]&&s2[i];i++) {
        if(s1[i]!=s2[i]) continue;
        if(flag==0) {if(s1[i]>=‘A‘&&s1[i]<=‘G‘) a=s1[i]-‘A‘+1,flag=1;}
        else if(flag==1){
            if(s1[i]>=‘0‘&&s2[i]<=‘9‘) b=s1[i]-‘0‘,flag=2;
            else if(s1[i]>=‘A‘&&s2[i]<=‘N‘) b=s1[i]-‘A‘+10,flag=2;
        }
    }
    for(int i=0;s3[i]&&s4[i];i++){
        if(s3[i]==s4[i]&&(s3[i]>=‘a‘&&s3[i]<=‘z‘)||(s3[i]>=‘A‘&&s3[i]<=‘Z‘)) {
            c=i;break;
        }
    }
    printf("%s%02d:%02d\n",f[a-1],b,c);
    return 0;
}