C :

#include <stdio.h>
#include <string.h>
#include<stdlib.h>
int main()
{
    int n,a[200],i ,N;
    char c,p[200];
    scanf("%d",&N);
    while(N--)
    {
        scanf("%s",p);
        if(p[0]>='0'&&p[0]<='9')
        {
            n = atoi(p);
            i = 0;
            while(n)
            {
                if(n%26==0)
                {
                    a[i++]= 'Z';
                    n-=26;
                    n/=26;
                }
                else
                {
                    a[i++]=64+n%26;
                    n/=26;
                }
            }
            i--;
            for(i; i>=0; --i)
                printf("%c",a[i]);
            printf("\n");
        }
        else
        {
            int len = strlen(p);
            int t1 = 0;
            for(i = 0 ; i < len ;i++)
            {
                int t = p[i]-'A'+1;
                t1 = t1*26 + t;
            }
            printf("%d\n",t1);
        }
    }
    return 0;
}

C++ :

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<cmath>
using namespace std;
int n,m,k,s,ans;
long  sum;
int main()
{
   char s[30];
   int a[30];
   int t,i;
   cin>>t;
   while(t--)
   {
       cin>>s;sum=0;
       int len=strlen(s),x;
       if(s[0]>'0' && s[0] <='9')
       {  x=0;
           for( i=0; i<len; i++)
           {
                sum=sum + s[i]-'0';
                     if(i<len-1)
                        sum=sum*10;
           }
          // cout<<sum<<endl;
           int kk=0;
           while(sum > 26)
           {
			   if(sum%26==0)
               {
				   a[kk]=26;
                    sum=sum/26-1;
			   }
			   else { a[kk]=sum%26;
			          sum=sum/26; }
			     kk++;
           }
           a[kk++]=sum;
         for( i=kk-1; i>=0; i--)
            if(a[i]==0)
                cout<<"Z";
              else printf( "%c", a[i]+'A'-1);
           printf("\n");
       }
       else
       {
           k=0;
           for( i=len-1 ; i>=0; i--)
           {
               long cot=pow(26,k);
               sum=cot*(s[i]-'A'+1)+sum;
                    k++;
           }
           cout<<sum<<endl;
       }
   }
   return 0;
}