3. Column Name from a Given Column Number

The problem can be found at the following link: Question Link

My Approach

To convert a given column number to an Excel column name, I follow these steps:

• Create a vector code of characters to represent the alphabet, where code[i] corresponds to the character for the column number i (0 to 25).

• Initialize the first element code[0] as 'Z' because in Excel, the last column is 'Z'.

• Iterate while n is greater than 0:

  • Append the character represented by code[n % 26] to the output string out.

  • If n % 26 is 0, decrement n by 1 to handle cases like column 26 ('Z').

  • Divide n by 26 to move to the next column.

• Reverse the out string to get the correct column name.

Time and Auxiliary Space Complexity

• Time Complexity: O(log(n)), The loop runs for the number of digits in n in base 26, which is equivalent to O(log(n)).

• Auxiliary Space Complexity: O(log(n)), The space required to store the output string out is proportional to the number of digits in n in base 26.

Code (C++)

class Solution {
public:
    string colName(long long int n) {
        string out;
        vector<char> code(26);
        for (int i = 1; i < 26; ++i)
            code[i] = (char)('A' + i - 1);
        code[0] = 'Z';

        while (n) {
            out += code[n % 26];
            if (n % 26 == 0)
                --n;
            n /= 26;
        }

        reverse(out.begin(), out.end());
        return out;
    }
};

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