Excel Sheet Column Number

EASY

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"].

Approaches

Checkout 2 different approaches to solve Excel Sheet Column Number. Click on different approaches to view the approach and algorithm in detail.

Recursive Approach

This method solves the problem by defining it in terms of itself. The column number for a title like "ABC" can be seen as the column number for "AB" multiplied by 26, plus the value of 'C'. This forms a recursive relationship that can be implemented with a function that calls itself on a smaller version of the input string.

Algorithm

Define a function titleToNumber(columnTitle).
Base Case: If columnTitle is empty, return 0.
Recursive Step:
- a. Get the prefix of the string (all characters except the last).
- b. Get the value of the last character (lastChar - 'A' + 1).
- c. Return titleToNumber(prefix) * 26 + value_of_last_char.

The core idea is to break down the problem. For a given columnTitle, we can separate the last character from the rest of the string (the prefix). The final number is 26 * (number for prefix) + (value of last character).

The base case for the recursion is an empty string, which corresponds to the number 0.
For any non-empty string, the function recursively calls itself with the prefix (the string without its last character) and uses the result to compute the final number.
For example, titleToNumber("ZY") would be calculated as titleToNumber("Z") * 26 + 25. The call to titleToNumber("Z") would in turn be titleToNumber("") * 26 + 26. Since titleToNumber("") is 0, the result unfolds back to (0 * 26 + 26) * 26 + 25 = 701.

class Solution {
    public int titleToNumber(String columnTitle) {
        // Base case: if the string is empty, its value is 0.
        if (columnTitle == null || columnTitle.isEmpty()) {
            return 0;
        }
        
        // Recursive step:
        // Get the prefix (all but the last character)
        String prefix = columnTitle.substring(0, columnTitle.length() - 1);
        // Get the last character
        char lastChar = columnTitle.charAt(columnTitle.length() - 1);
        
        // The value of the last character (A=1, B=2, ...)
        int lastCharValue = lastChar - 'A' + 1;
        
        // The recursive formula
        return titleToNumber(prefix) * 26 + lastCharValue;
    }
}

Complexity Analysis

Time Complexity: O(N^2)Space Complexity: O(N^2)

Pros and Cons

Pros:

Provides a clear, declarative solution that directly models the mathematical recurrence relation.

Cons:

Inefficient due to the overhead of recursive calls and repeated string manipulations (like substring).
Can lead to a StackOverflowError for very long strings, although not an issue with the problem's constraints.
Higher space complexity due to the recursion stack.

Code Solutions

Checking out 4 solutions in different languages for Excel Sheet Column Number. Click on different languages to view the code.

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

Video Solution

Watch the video walkthrough for Excel Sheet Column Number

Patterns:

Math

Data Structures:

String

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