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171. Excel Sheet Column Number

Description

Given a string columnTitle that represents the column title as appears in an Excel sheet, return its corresponding column number.

For example:

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

 

Example 1:

Input: columnTitle = "A"
Output: 1

Example 2:

Input: columnTitle = "AB"
Output: 28

Example 3:

Input: columnTitle = "ZY"
Output: 701

 

Constraints:

  • 1 <= columnTitle.length <= 7
  • columnTitle consists only of uppercase English letters.
  • columnTitle is in the range ["A", "FXSHRXW"].

Solutions

Solution 1: Base Conversion

Thinking

Title to number is the inverse of problem \(168\): each letter is a digit in \(1\ldots 26\). Fold left to right with \(\textit{ans}=\textit{ans}\times 26 + (c-'A'+1)\), the same pattern as parsing a decimal string.

The column name in Excel is a representation in base 26. For example, "AB" represents the column number \(1 \times 26 + 2 = 28\).

Therefore, we can iterate through the string columnTitle, convert each character to its corresponding value, and then calculate the result.

The time complexity is \(O(n)\), where \(n\) is the length of the string columnTitle. The space complexity is \(O(1)\).

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

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class Solution {
    public int titleToNumber(String columnTitle) {
        int ans = 0;
        for (int i = 0; i < columnTitle.length(); ++i) {
            ans = ans * 26 + (columnTitle.charAt(i) - 'A' + 1);
        }
        return ans;
    }
}

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class Solution {
public:
    int titleToNumber(string columnTitle) {
        int ans = 0;
        for (char& c : columnTitle) {
            ans = ans * 26 + (c - 'A' + 1);
        }
        return ans;
    }
};

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func titleToNumber(columnTitle string) (ans int) {
    for _, c := range columnTitle {
        ans = ans*26 + int(c-'A'+1)
    }
    return
}

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function titleToNumber(columnTitle: string): number {
    let ans: number = 0;
    for (const c of columnTitle) {
        ans = ans * 26 + (c.charCodeAt(0) - 'A'.charCodeAt(0) + 1);
    }
    return ans;
}

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public class Solution {
    public int TitleToNumber(string columnTitle) {
        int ans = 0;
        foreach (char c in columnTitle) {
            ans = ans * 26 + c - 'A' + 1;
        }
        return ans;
    }
}

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