You are given two strings, key and message, which represent a cipher key and a secret message, respectively.

To decode message, construct a substitution table using key according to the following steps:

1. Traverse key from left to right and take the first appearance of each unique lowercase English letter.

2. Map these unique letters in order to the standard $26$ lowercase English letters, mapping the first unique character to 'a', the second to 'b', the third to 'c', and so on up to 'z'.

3. Spaces ' ' are not mapped and remain as spaces ' '.

Using the substitution table, decode every character in message and return the resulting decoded string.

Constraints:

• $26 \leq$ key.length $\leq 2000$

• key consists of lowercase English letters and ' '.

• key contains every letter in the English alphabet ('a' to 'z') at least once.

• $1 \leq$ message.length $\leq 2000$

• message consists of lowercase English letters and ' '.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.