What is the time complexity of Decode the Message?

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

What data structures are used to solve Decode the Message?

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

What are the key insights for solving Decode the Message?

The problem requires creating a mapping from the characters in the key to their corresponding decoded characters. A hash table (or similar data structure) is ideal for efficient lookups. The order of letters in the key is crucial; only the first occurrence of each letter matters. Subsequent occurrences are ignored.

Which companies ask LeetCode 2325. Decode the Message in interviews?

This problem is frequently asked at Coinbase, Tesla.

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Decode the Message - LeetCode 2325 Solution

Decode the Message - Complete Solution Guide

Decode the Message is LeetCode problem 2325, 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

You are given the strings key and message , which represent a cipher key and a secret message, respectively. The steps to decode message are as follows: Use the first appearance of all 26 lowercase English letters in key as the order of the substitution table. Align the substitution table with the regular English alphabet. Each letter in message is then substituted using the table. Spaces ' ' are transformed to themselves. For example, given key = " hap p y bo y" (actual key would have at least

Detailed Explanation

The problem asks you to decode a secret message using a given cipher key. The key determines a substitution cipher: the first occurrence of each letter in the key maps to the next letter in the alphabet (a -> a, b -> b, etc.). Spaces remain spaces. The input consists of the key (a string containing all 26 letters at least once) and the message (a string to decode). The output is the decoded message.

Solution Approach

The provided solutions all use a similar approach: they iterate through the key, creating a mapping from each unique character (excluding spaces) to its corresponding decoded character. Then, they iterate through the message, using the mapping to decode each character. The result is built character by character into a new string.

Step-by-Step Algorithm

Initialize a mapping data structure (hash table/array).
Iterate through the key string:
If a character is not a space and not already in the mapping:
Add the character to the mapping, assigning it the next letter in the alphabet.
Iterate through the message string:
If the character is a space, append a space to the decoded message.
Otherwise, look up the character in the mapping and append its decoded character to the decoded message.
Return the decoded message.

Key Insights

The problem requires creating a mapping from the characters in the key to their corresponding decoded characters. A hash table (or similar data structure) is ideal for efficient lookups.
The order of letters in the key is crucial; only the first occurrence of each letter matters. Subsequent occurrences are ignored.
Handling spaces correctly is important; they are not part of the substitution mapping.

Complexity Analysis

Time Complexity: O(m+n)

Space Complexity: O(1)

Topics

This problem involves: Hash Table, String.

Companies

Asked at: Coinbase, Tesla.

Decode the Message | LeetCode 2325

Easy

Hash Table String

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

Solution Code

Available in:

class Solution {
    public String decodeMessage(String key, String message) {
        char[] decode = new char[26];
        int index = 0;
        for (char c : key.toCharArray()) {
            if (c != ' ' && decode[c - 'a'] == '\0') {
                decode[c - 'a'] = (char) ('a' + index);
                index++;
            }
        }

        StringBuilder decodedMessage = new StringBuilder();
        for (char c : message.toCharArray()) {
            if (c == ' ') {
                decodedMessage.append(' ');
            } else {
                decodedMessage.append(decode[c - 'a']);
            }
        }

        return decodedMessage.toString();
    }
}

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Problem Info

Description

Problem 2325: Decode the Message - Detailed explanation and solution approaches.

Topics

Hash Table String

Companies

CoinbaseTesla

Stats

Difficulty:Easy

Languages:4

Companies:2

Problem #:2325

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

8 steps

1
Initialize a mapping data structure (hash table/array).
2
Iterate through the key string:
3
If a character is not a space and not already in the mapping:
4
Add the character to the mapping, assigning it the next letter in the alphabet.
5
Iterate through the message string:
6
If the character is a space, append a space to the decoded message.
7
Otherwise, look up the character in the mapping and append its decoded character to the decoded message.
8
Return the decoded message.

💡

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(m+n)

💾

Space Complexity

O(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The first loop has a maximum number of iterations equal to the length of the key. The second loop has a maximum number of iterations equal to the length of the message. The space used to store the mapping is constant because there are only 26 letters in the alphabet. The output string uses space proportional to the length of the message, but is typically considered output space and not factored into the algorithmic space complexity.

📖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

Using a 26-element array for mapping

Instead of a hash table, a 26-element array can be used. The index of the array corresponds to the character's position in the alphabet (a=0, b=1, ...). This is slightly more space-efficient than a hash table but may require more careful indexing.

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

Trade-offs: The array approach is slightly faster in some cases due to direct access, but the hash table approach is more flexible and suitable if you need to handle more than just 26 letters.

✨

Key Insights

1The problem requires creating a mapping from the characters in the key to their corresponding decoded characters. A hash table (or similar data structure) is ideal for efficient lookups.
2The order of letters in the key is crucial; only the first occurrence of each letter matters. Subsequent occurrences are ignored.
3Handling spaces correctly is important; they are not part of the substitution mapping.

📚

Key Terms Explained

For Students

Hash Table

A data structure that maps keys to values for quick lookup. Like a dictionary!

String

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

Edge Cases

Empty message: The solution handles this correctly; an empty message results in an empty decoded message.
Message with only spaces: The solution correctly handles this; it outputs a string containing only spaces.

⚠️

Common Mistakes to Avoid

✗Incorrectly handling the first occurrence of letters in the key: It's crucial to only use the first occurrence of each letter. Using subsequent occurrences will lead to an incorrect mapping.

✗Forgetting to handle spaces: Spaces must be treated as spaces and not mapped to any letter.

✅

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?

💼Cryptography: Simple substitution ciphers are a fundamental building block in more complex cryptographic systems. Understanding this concept is essential for learning about more advanced encryption and decryption techniques.

🏦Data encoding/decoding: Similar substitution methods are used in various data encoding schemes to represent data compactly or in a secure way.

💡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

Hash tables trade space for time. Before using one, consider if the problem constraints allow the extra memory. In interviews, always mention this trade-off.

🎯 Interview Tip

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

⚠️ Watch Out

Don't forget to handle key collisions and null/undefined keys. Consider what happens with duplicate values.

👨‍💻 From Experience

This is one of the first problems I solved when learning Hash Table. It's a great foundation for more complex problems.

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 13, 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

Don't forget to handle the edge case when the input is empty. Many candidates miss this in interviews.

@DevExpert92Jul 14

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

@AlgoMasterJul 7

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

@InterviewProJun 30

❓

Frequently Asked Questions

How do I choose between a hash map and a hash set?

Use a hash set when you only need to track presence/absence of elements (like detecting duplicates). Use a hash map when you need to associate values with keys (like counting frequencies or storing indices). Hash sets use less memory since they only store keys.

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 "Decode the Message"?

For this easy-level Hash Table 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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