Krdytkyu

Excel column letters to actual numbers, if you recall, Excel names its columns with letters from A to Z, and then the sequence goes AA, AB, AC... AZ, BA, BB, etc.
You have to write a function that accepts a string as a parameter (like "AABCCE") and returns the actual column number. And then do the exact reverse, given column number return the column name.

Also verify complexity: $O(log n)$, where $n$ is the input number while $log$ is to base the base being considered (hexa, decimal or binary etc.).

public final class Excel {



    private Excel() {}  



    public static int getExcelColumnNumber(String column) {

        int result = 0;

        for (int i = 0; i < column.length(); i++) {

            result *= 26;

            result += column.charAt(i) - 'A' + 1;

        }

        return result;

    }



    public static String getExcelColumnName(int number) {

        final StringBuilder sb = new StringBuilder();



        int num = number - 1;

        while (num >=  0) {

            int numChar = (num % 26)  + 65;

            sb.append((char)numChar);

            num = (num  / 26) - 1;

        }

        return sb.reverse().toString();

    }





    public static void main(String args) {

         Assert.assertEquals(53, getExcelColumnNumber("BA"));

         Assert.assertEquals("BA", getExcelColumnName(53));



         Assert.assertEquals(703, getExcelColumnNumber("AAA"));

         Assert.assertEquals("AAA", getExcelColumnName(703));



         Assert.assertEquals(26, getExcelColumnNumber("Z"));

         Assert.assertEquals("Z", getExcelColumnName(26));



         Assert.assertEquals(702, getExcelColumnNumber("ZZ"));

         Assert.assertEquals("ZZ", getExcelColumnName(702));



    }

}

Your code is basically fine, and your unit tests are good. All I have is nitpicks.

"Excel" in the method names are a bit redundant.

In getExcelColumnNumber(), group - 'A' + 1 with parentheses as - ('A' - 1). Then the compiler can generate 64 as a constant.

In getExcelColumnName(), the similarly named variables number, num, and numChar are confusing.

The complexity O(log n) is correct. (With Big-O notation, the base of the logarithm is an unimportant detail, since the base just scales the logarithm by a constant factor, and constant factors are conventionally discarded with Big-O. For example, O(log_26 n) = O(ln n / ln 26) = O(ln n).)

public final class ExcelColumn {



    private ExcelColumn() {}



    public static int toNumber(String name) {

        int number = 0;

        for (int i = 0; i < name.length(); i++) {

            number = number * 26 + (name.charAt(i) - ('A' - 1));

        }

        return number;

    }



    public static String toName(int number) {

        StringBuilder sb = new StringBuilder();

        while (number-- > 0) {

            sb.append((char)('A' + (number % 26)));

            number /= 26;

        }

        return sb.reverse().toString();

    }

}

Here's the recursive variant:

static String toName( final int i )

{

     if ( i < 26 ) return Character.toString( (char)(65+i) );

     return toName( (i/26)-1 ) + toName( i%26 );

}

It's more elegant, perhaps.