Decode the Message

Imagine you have two sets of English alphabets - one is in the regular order (a, b, c, …, z) and the other is shuffled (for example, “thequickbrownfxjmpsvlazydg”).

Now, you receive a secret message. Each letter in this message was originally written using the regular order of English alphabets, but then it was changed (or encrypted) using the shuffled order of alphabets.

Your task is to convert the secret message back into regular English. You’ll do this by replacing each letter in the secret message with the corresponding letter from the regular English alphabets, based on its position in the shuffled alphabet.

In short, you’re trying to decode a message that was encoded using a custom order of alphabets (provided in the key) instead of the regular ‘a’ to ‘z’.

One more thing to note is that the space character (’ ‘) remains the same in both the encoded and decoded message.

Solution

The problem can be solved by creating a mapping of each character from the key to the corresponding character in the English alphabet (in the order they appear in the key). Then we apply this mapping to the message.

Here is the Python code implementing this approach:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14

class Solution:
    def decodeMessage(self, key: str, message: str) -> str:
        mapping = {}
        alphabet = 'abcdefghijklmnopqrstuvwxyz'
        idx = 0

        for char in key:
            if char not in mapping and char != ' ':
                mapping[char] = alphabet[idx]
                idx += 1

        decoded_message = ''.join([mapping[char] if char != ' ' else ' ' for char in message])

        return decoded_message

In this code, we first initialize an empty dictionary to store our character mappings. We also initialize a string alphabet containing all English lowercase letters and an index idx to track the current letter in the alphabet.

Next, we iterate over each character in the key. If the character is not a space and has not been encountered before, we map it to the current letter in the alphabet and increment idx.

Finally, we generate the decoded message by substituting each character in the message using the mapping (preserving spaces), and return the decoded message.

Abstract Representation of the Problem

At a high level, this problem is an instance of a substitution cipher, which is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system. The receiver deciphers the text by performing the inverse substitution.

To abstract the problem, consider that we have a source alphabet and a target alphabet. Each unique character in the source alphabet is mapped to a unique character in the target alphabet based on the order they appear in the source.

We have two inputs: a key (source) and a message (encrypted text). The goal is to use the mapping determined by the key to transform the message from the target alphabet back to the source alphabet.

It’s important to note that this problem includes a special character (space) that is mapped to itself.

In terms of data, we’re working with strings and mappings between sets of characters, and the primary operation is substitution based on that mapping.

This abstract problem can be applied in various contexts beyond encryption, including data transformation and encoding/decoding problems.

Q&A

we can solve this using Concurrent hashmap also but why putifabsent method is not working for this??

The putIfAbsent method in a ConcurrentHashMap is designed to put a key-value pair into the map only if the key is not already present.

In the context of this problem, you want to establish a mapping from every character in the shuffled alphabet (key) to its corresponding character in the regular alphabet. However, you don’t want to overwrite any existing mappings.

The putIfAbsent method could fail in this case if the same character appears multiple times in the shuffled alphabet, because it would ignore all occurrences of the character after the first one. Since the problem statement specifies that the key will contain each letter of the alphabet at least once, this means that any extra occurrences of a character in the key would not be considered.

So in short, putIfAbsent isn’t suitable for this problem because it’s possible that the same character may appear more than once in the shuffled alphabet (key), and putIfAbsent would not create the correct mappings for these extra occurrences.