strstr

const char* strstr(const char *str, const char* substr)
{
    int i, j, temp;

    for (i = 0; str[i] != ‘\0‘; i++)
    {
        j = 0;
        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;
}