Excel Column Name

main.c

#include <stdio.h>
#include <math.h>

#define AtoZ 26
#define ASCII_A 65

int calc( char* c, int* sum )
{
	if ( *c == '\0' )
	{
		return 0;
	}
	int digit = calc( c + 1, sum );
	int a = (int)*c - ASCII_A + 1;
	*sum += a * pow( AtoZ, digit );
	return digit + 1;
}

int main( int argc, char* argv[] )
{
	char* row = argv[ 1 ];
	int result = 0;
	calc( row, &result );
	printf( "%d\n", result );
}

main2.c

#include <stdio.h>

int calc( char* c, int sum )
{
	if ( *c == '\0' )
	{
		return sum;
	}
	int a_to_z    = 'Z' - 'A' + 1;
	int now_digit = (int)*c - 'A' + 1;
	return calc( c + 1, sum * a_to_z + now_digit );
}

int main( int argc, char* argv[] )
{
	printf( "%d\n", calc( argv[ 1 ], 0 ) );
}