自定义strcmp,strlen,strcpy,strcat函数

char *strcpy(char *s1,const char *s2){
    char *p=s1;
    while(*p++=*s2++);
    return s1;
}
size_t strlen(const char *s){
    size_t n=0;
    while(*s++)n++;
    return n;
}

char *strcat(char *s1,const char *s2){
    char *p=s1;
    while(*p)p++;
    while(*p++=*s2++);
    return s1;
}

int strcmp(const char *s1,const char *s2){
    while(*s1==*s2){
        if(*s1=='\0')return 0;
        s1++;s2++;
    }
    return *s1-*s2;
}