This programming problem deals with the Linked List (LLStack) implementation.

a. Create a simple user interface to allow a user to input an expression and store it as a string variable.

b. Write a function using the LLStack implementation to check if the input string is a well-formed parenthetical expression. Assume (), {}, and [] are all parenthetical characters. Recall that a parenthetical expression is well-formed if

i. All closing parentheses match the most recent, unmatched opening parentheses of the same type.

ii. All opening parentheses have matching closing parentheses of the same type by the end of the expression.

The function should return true if the expression is well-formed, and false otherwise.