strstr实现

char * strstr(char *buf, char *sub)
{
    char *bp;
    char *sp;
    if(buf==NULL || sub ==NULL){
        return 0;
    }
    if(!(*sub)){
        return buf;
    }
    while(*buf)
    {
        bp = buf;
        sp = sub;
        do{
            if(!(*sp))
            return buf;
        } while(*(bp++) == *(sp++));
        buf += 1;
    }
    return 0;
}