You could have a stack of (paren-type, integer) pairs. If the paren has a different type than the top pair on the stack (or the stack is empty), push a new pair

You could have a stack of (paren-type, integer) pairs. If the paren has a different type than the top pair on the stack (or the stack is empty), push a new pair with the integer set to one. If the integer goes to zero, pop the pair. At the end of the expression, you have to have an empty stack.