168. Excel Sheet Column Title - Explanation

Problem Link

Description

You are given an integer columnNumber, return its corresponding column title as it appears in an Excel sheet.

For example:

A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28 
...

Example 1:

Input: columnNumber = 32

Output: "AF"

Example 2:

Input: columnNumber = 53

Output: "BA"

Constraints:

• 1 <= columnNumber <= ((2^31)-1)

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1. Recursion

class Solution:
    def convertToTitle(self, columnNumber: int) -> str:
        if columnNumber == 0:
            return ""

        n = columnNumber - 1
        return self.convertToTitle(n // 26) + chr(n % 26 + ord('A'))

Time & Space Complexity

• Time complexity: $O(\log n)$
• Space complexity: $O(\log n)$ for recursion stack.

Where $n$ is the given column number.

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2. Iteration

class Solution:
    def convertToTitle(self, columnNumber: int) -> str:
        res = []
        while columnNumber > 0:
            columnNumber -= 1
            offset = columnNumber % 26
            res += chr(ord('A') + offset)
            columnNumber //= 26

        return ''.join(reversed(res))

Time & Space Complexity

• Time complexity: $O(\log n)$
• Space complexity: $O(1)$ extra space.

Where $n$ is the given column number.