#include <stdio.h>
#include <stdlib.h>

int isValid(char* str)
{
    int flag = 0;
    if (*str >= '0' && *str <= '9')
        return 0;
    while (*str)
    {
        if (flag == 0)
        {
            if (!((*str >= 'a' && *str <= 'z') || (*str >= 'A' && *str <= 'Z') || (*str >= '0' && *str <= '9')))
            {
                return 0;
            }
            if ((*str >= '0' && *str <= '9'))
                flag = 1;
        }
        str++;
    }
    if (flag == 0)
        return 0;
    else
        return 1;
}

char* fun(char* str)
{
    int count = 0;
    char* tmp,*num,*newStr;
    tmp = str;
    if (isValid(str) == 0)
        return NULL;
    while (*tmp != NULL && !(*tmp >= '0' && *tmp <= '9'))
    {
        count++;
        tmp++;
    }
    //tmp points to the desired num
    num = tmp;
    newStr = (char*)calloc(((int)(*num - '0') * count) + 1, sizeof(char));
    if (!newStr)
        exit(1);
    tmp = str;
    for (int i = 0,k=0; i < (int)(*num - '0'); i++)
    {
        for (int j = 0; j < count; j++)
        {
            *(newStr + (k++)) = *(tmp + j);
        }
    }
    return newStr;
}

void main()
{
    char* str = "Good567Day",*ptr;
    ptr = fun(str);
}