The essential feature is that when a closing delimiter is encountered, it should be a match for the most recent opening delimter.

Using this fact ("most recent"), a stack holding the opening delimiters can be used:

1. When a character is read, just print it.

2. If the character is an opening delimiter, push it on the stack.

3. If the character is a closing delimiter, check that is a match for the top element of the stack (the most recent opening delimiter).

   If there is a match, just pop the opening delimiter off the stack, otherwise the input is not balanced.

Some details:

What if the stack is not empty after all the input has been read?

What if a closing delimiter is read, but the stack is empty?

(See hw2.)