Under the specified grammar, an expression evaluates to a set of strings:

• A single lowercase letter evaluates to a set containing just that character.

• The union of two or more expressions enclosed in curly braces {e_1, e_2, ...} evaluates to the union of the sets generated by $e_1, e_2, \dots$.

• The concatenation of two expressions $e_1 e_2$ evaluates to the set formed by concatenating every word from $e_1$ with every word from $e_2$.

Given a string expression representing a valid grammar expression, return the sorted list of all unique words generated by the expression.

Constraints:

• $1 \leq$ expression.length $\leq 60$

• expression[i] consists of '{', '}', ',', or lowercase English letters.

• The given expression represents a set of words based on the grammar given in the description.

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.