What is the time complexity of Excel Sheet Column Number?

The optimal solution has a time complexity of O(n).

What data structures are used to solve Excel Sheet Column Number?

This problem uses Math, String. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Excel Sheet Column Number?

Insight 1: Recognizing that the column numbering system is essentially a base-26 number system where A=1, B=2, ..., Z=26. Insight 2: Understanding that we can iterate through the column title string from left to right, multiplying the accumulated result by 26 for each character and adding the character's value.

Which companies ask LeetCode 171. Excel Sheet Column Number in interviews?

This problem is frequently asked at Docusign, Zoho, razorpay.

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Excel Sheet Column Number - LeetCode 171 Solution

Q: How to solve Excel Sheet Column Number?

The solution uses a iterative approach. It traverses the input string character by character. For each character, it calculates its numerical value (1 for 'A', 2 for 'B', ..., 26 for 'Z') and adds this value to the running total after multiplying the total by 26. This multiplication by 26 effectively shifts the place value in the base-26 system, similar to multiplying by 10 when working in base-10.

Excel Sheet Column Number - Complete Solution Guide

Excel Sheet Column Number is LeetCode problem 171, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

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"] .

Detailed Explanation

The problem asks you to convert an Excel-style column title (e.g., "A", "AB", "ZY") into its corresponding column number. The columns are numbered sequentially starting from 1: A=1, B=2, ..., Z=26, AA=27, AB=28, and so on. The input is a string containing only uppercase English letters, and the output is an integer representing the column number. The constraints specify that the input string length is between 1 and 7 characters, and it falls within a specific range.

Solution Approach

The solution uses a iterative approach. It traverses the input string character by character. For each character, it calculates its numerical value (1 for 'A', 2 for 'B', ..., 26 for 'Z') and adds this value to the running total after multiplying the total by 26. This multiplication by 26 effectively shifts the place value in the base-26 system, similar to multiplying by 10 when working in base-10.

Step-by-Step Algorithm

Step 1: Initialize a variable `result` to 0. This variable will store the final column number.
Step 2: Iterate through the characters of the input string `columnTitle`.
Step 3: For each character, calculate its value: `char_value = ord(char) - ord('A') + 1`. This subtracts the ASCII value of 'A' from the character's ASCII value and adds 1, effectively mapping 'A' to 1, 'B' to 2, and so on.
Step 4: Update `result`: `result = result * 26 + char_value`. This performs the base-26 conversion.
Step 5: After iterating through all characters, return `result`.

Key Insights

Insight 1: Recognizing that the column numbering system is essentially a base-26 number system where A=1, B=2, ..., Z=26.
Insight 2: Understanding that we can iterate through the column title string from left to right, multiplying the accumulated result by 26 for each character and adding the character's value.
Insight 3: The use of ASCII values (ord() in Python) to efficiently convert characters to their numerical representation is crucial for an optimal solution.

Complexity Analysis

Time Complexity: O(n)

Space Complexity: O(1)

Topics

This problem involves: Math, String.

Companies

Asked at: Docusign, Zoho, razorpay.

Excel Sheet Column Number | LeetCode 171

Easy

Math String

Time: O(n)Space: O(1)

Solution Code

Available in:

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

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Problem Info

Description

Problem 171: Excel Sheet Column Number - Detailed explanation and solution approaches.

Topics

Math String

Companies

DocusignZohorazorpay

Stats

Difficulty:Easy

Languages:4

Companies:3

Problem #:171

Problem Explanation

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

📋

Step-by-Step Solution

5 steps

1
Step 1: Initialize a variable `result` to 0. This variable will store the final column number.
2
Step 2: Iterate through the characters of the input string `columnTitle`.
3
Step 3: For each character, calculate its value: `char_value = ord(char) - ord('A') + 1`. This subtracts the ASCII value of 'A' from the character's ASCII value and adds 1, effectively mapping 'A' to 1, 'B' to 2, and so on.
4
Step 4: Update `result`: `result = result * 26 + char_value`. This performs the base-26 conversion.
5
Step 5: After iterating through all characters, return `result`.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(n)

👍 Linear - Good! Visits each element once.

💾

Space Complexity

O(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The time complexity is linear because the number of operations is directly proportional to the length of the input string. The space complexity is constant because only a few variables are used, irrespective of the input string's length.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

Recursive Approach

A recursive solution could also be implemented, where each recursive call processes one character and calculates its contribution to the column number. The base case would be an empty string.

Time: O(n)Space: O(n) due to the recursive call stack

Trade-offs: The recursive approach is less efficient in terms of space complexity due to the function call overhead. The iterative approach is generally preferred for its better space efficiency.

✨

Key Insights

1Insight 1: Recognizing that the column numbering system is essentially a base-26 number system where A=1, B=2, ..., Z=26.
2Insight 2: Understanding that we can iterate through the column title string from left to right, multiplying the accumulated result by 26 for each character and adding the character's value.
3Insight 3: The use of ASCII values (ord() in Python) to efficiently convert characters to their numerical representation is crucial for an optimal solution.

📚

Key Terms Explained

For Students

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Edge case 1: The input string is a single character (e.g., "A", "Z"). The algorithm correctly handles this by performing the calculation for a single character and returning the appropriate result.
Edge case 2: The input string is empty. Although the problem constraints prevent this, a robust solution would handle it by returning 0 or raising an error.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Incorrectly calculating the character value. Off-by-one errors are common when mapping characters to numbers. Ensuring correct handling of ASCII values is critical.

✗Common mistake 2: Forgetting to multiply the accumulated result by 26 in each iteration. This would lead to incorrect column numbers.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Real-world scenario 1: Spreadsheet software often needs to convert between column titles and column indices for internal data management and addressing.

🏦Real-world scenario 2: Data analysis tools that use Excel-like data structures might employ this type of conversion for processing or displaying data based on column identifiers.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

When dealing with arrays, always consider if sorting would simplify the problem. Many array problems have O(n) solutions using hash maps or two-pointer technique.

🎯 Interview Tip

Even for easy problems, discuss time and space complexity. Interviewers want to see you think analytically.

⚠️ Watch Out

Always test your solution with edge cases: empty input, single element, and boundary values.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: November 19, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

The key insight here is recognizing the pattern. Once you see it, the solution becomes straightforward.

@AlgoMasterJul 13

I was asked this exact problem at my Google interview. The interviewer wanted me to optimize space complexity.

@InterviewProJul 6

Try solving this without using extra space first. It's a great exercise for understanding the underlying pattern.

@CodeNinjaJun 29

❓

Frequently Asked Questions

What is the sliding window technique for string problems?

Sliding window maintains a "window" of characters with two pointers. Expand the right pointer to include characters, shrink from left when window violates constraints. Useful for substring problems, anagrams, and longest/shortest substring with properties.

What should I learn from easy problems?

Easy problems introduce core patterns that appear in harder problems. Master basic operations (iteration, conditionals, simple data structures), recognize common patterns (counting, searching, basic transformations), and practice explaining your thought process clearly.

What is the importance of time and space complexity analysis?

Complexity analysis is crucial because: 1) Interviewers always ask about it, 2) It helps you choose between approaches, 3) It demonstrates CS fundamentals. Always state both time and space complexity, and be prepared to explain how you derived them.

What is the optimal approach for "Excel Sheet Column Number"?

For this easy-level Math problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether string techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

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