P3.1 Stack
--
example of stack
 stack: The stack of trays in a cafeteria; Back/Forward stacks on browsers;

stack: LIFO
push and pop
variable: only one (index top is enough)

example: push(a); push(b); push(c); push(d)
top->	d
	c
	b
	a

if start from -1, then now top=3
# note: push need one argument, pop no need argument, however it has return vale

boundary conditions: overflow and empty
empty: top == -1
overflow: top == MAX 

#note: push need to check overflow and pop need to check empty

coding: