Stack
Stack is a container of elements that are inserted and deleted according to LIFO (Last In First Out).
For n stack Si (i=0,1,...,n−1), perform a sequence of the following operations.
push(t, x): Insert an integer x to St.
top(t): Report the value which should be deleted next from St. If St is empty, do nothing.
pop(t): Delete an element from St. If St is empty, do nothing.
In the initial state, all stacks are empty.