Practice: Stacks

Four problems for the stack: brackets, letters that cancel, a postfix calculator and a stack that knows its minimum.

• Check empty() before every top() and pop(), unless the statement promises the stack can't be empty there.
• The strings go up to 2·10⁵ characters: one pass with a stack, never a search for each bracket's partner.
• For the letters that vanish in pairs, keep a stack<char>: what is left on it at the end is the answer. Pour it into a second stack to print it in order.
• In the postfix calculator, read every token as a string: -5 is a number, and only - on its own is an operator. Keep long long on the stack.
• Monotonic Stack is the stack that knows its minimum. Its time limit is tight, so keep the fast-IO lines.