# 3. Column Name from a Given Column Number

The problem can be found at the following link: [Question Link](https://practice.geeksforgeeks.org/problems/column-name-from-a-given-column-number4244/1)

## 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++)

```cpp
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;
    }
};
```

## Contribution and Support

For discussions, questions, or doubts related to this solution, please visit our [discussion section](https://github.com/getlost01/gfg-potd/discussions). We welcome your input and aim to foster a collaborative learning environment.

If you find this solution helpful, consider supporting us by giving a `⭐ star` to the [getlost01/gfg-potd](https://github.com/getlost01/gfg-potd) repository.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://gl01.gitbook.io/gfg-editorials/2023/10-2023-oct-2/03-column-name-from-a-given-column-number.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.