幾個筆試必備string函數（已經調過）

char *mystrchr(char *src, int letter)
{
    assert(src != NULL);
    while((*src != letter)&&(*src))
        src++;
    if(*src=='\0')
        return NULL;
    return (src);
}


char *mystrcat(char *des,const char *src)
{
    assert((des != NULL) && (src !=NULL));
    char *address = (char *)malloc(strlen(des)+strlen(src)+1);
	char *p_address = address;
	while((*address = *des) != '\0')
	{
		++address;
		++des;
	}
	while((*address++ = *src++) != '\0');//已經賦值'\0'再判斷!='\0'
    return p_address;
}



void  *my_memcpy(void *Des, const void *Src, int len)
{
    assert((Des != NULL) && (Src != NULL));
    void *address = Des;
    while(len--)
        *(char *)Des++ = *(char *)Src++;
    return address;
}

int strcmp(const char *s, const char *t)
{
    assert(s != NULL && t != NULL);
    while (*s && *t &&( *s++ == *t++));
    return (*s - *t);
}

char *strcpy(char *strDes, const char *strSrc)
{
    assert((strDes != NULL) && (strSrc != NULL));
    char *address = strDes;
    while ((*strDes ++ = *strSrc ++) != '\0');
    return address;
}