LeetCode 171: Excel Sheet Column Number

Meta Data

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Difficulty: easy First Attempt: 2022-08-21

• Submissions: Runtime Error → Accepted ×7

Approach

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The reverse of LC 168 (Math Template 7): each letter is worth ord(c) - ord("A") + 1, and scanning left to right with res = res * 26 + value works just like parsing a decimal string.

My Attempts

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Attempt 1 wrote for i, n in columnTitle[::-1] without enumerate, so unpacking failed (ValueError). Adding it passed.

Solution

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class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
        res = 0
        for c in columnTitle:
            res = res * 26 + ord(c) - ord("A") + 1
        return res

Complexity

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Time O(n), space O(1).

Mistakes to Avoid

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• A is 1, not 0.
• Use enumerate when both the index and the character are needed.

Interview Explanation

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Scanning left to right, I multiply the running result by 26 and add the letter’s value, with A as 1. Time O(n).