do{if(*p==a)*p=b;}while(*(++p)!='\0);
while(*p&&(*p==a&&((*p=b)||1))&&p++);
while(p[i]){if(p[i]==a)p[i]=b;i++;}
for(;*p!='\0';p++)if(*p==a)*p=b;
for(;*p;p++)*p==a&&(*p=b);
while(*p=*p==a?b:*p)p++;

第一次short program contestヽ(´ー｀)ノお疲れ様