How should I practice Double Substitution Decryption?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Upstart online assessment problems in FastPrep.

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Problem · String

Double Substitution Decryption

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UpstartFULLTIMEOA

Problem statement

A message was transformed by two one-to-one character substitutions:

Alice's characters were substituted into Bob's characters.
Bob's characters were substituted into Carol's characters.

You receive aligned clear/cipher training text for each stage:

alicePlain and bobCipher define the Alice-to-Bob mapping.
bobPlain and carolCipher define the Bob-to-Carol mapping.

When learning either mapping, remove all whitespace from both aligned strings before pairing their characters. The non-whitespace strings have equal lengths and consistently define one-to-one mappings.

Decrypt encryptedMessage from Carol's alphabet back to Bob's alphabet, then from Bob's alphabet back to Alice's alphabet. Preserve every whitespace character in encryptedMessage unchanged. Return the fully decrypted message.

Function

decryptDoubleSubstitution(alicePlain: String, bobCipher: String, bobPlain: String, carolCipher: String, encryptedMessage: String) → String

Examples

Example 1

alicePlain = "abc"bobCipher = "bcd"bobPlain = "bcd"carolCipher = "cde"encryptedMessage = "edc cde"return = "cba abc"

Reversing the Carol-to-Bob mapping changes edc cde to dcb bcd. Reversing the Bob-to-Alice mapping then produces cba abc.

Example 2

alicePlain = "a b c"bobCipher = "x y z"bobPlain = "x y z"carolCipher = "m n o"encryptedMessage = "mno onm"return = "abc cba"

Whitespace in the training text does not create mapping entries. The single space in the encrypted message is preserved.

Constraints

Each input string has length from 1 through 10^5.
After whitespace removal, each training pair has equal length.
Each training pair defines a consistent one-to-one mapping.
Every non-whitespace character needed while decrypting is covered by the corresponding inverse mapping.
Characters are case-sensitive.