const char* strstr(const char *str, const char* substr)

{

    int i, j, temp;

    for (i = ; str[i] != '\0'; i++)

    {

        j = ;

        temp = i; //记录当前主字符串位置

        while (str[i++]==substr[j++])

        {

            if (substr[j] == '\0')

            {

                return &str[temp];

            }

            else if (str[i]=='\0')

            {

                return NULL;

            }

        }

    }

    return NULL;

}

int strcmp(const char* str1, const char* str2)

{

    while (*str1==*str2)

    {

        if (*str1 == '\0')

        {

            return ;

        }

        ++str1; ++str2;

    }

    return *str1 - *str2;

}

unsigned int strlen(const char* str)

{

    const char* res = str;

    while (*res++); //res跳出循环后,仍执行++

    return (res - str - );

}

char* strcpy(char *dst,const char* src)

{

    assert(==dst||==src);

    char* tmp = dst;

    while (*src!='')

    {

        *dst++ = *src++;

    }

    *dst = '\0';

    return tmp;

}