A Stack Class

A stack is a pile where the last thing you put on is the first thing you take off, like a pile of plates. Build a Stack class with these methods:

• Stack() makes an empty stack.
• push(item) puts item on top.
• pop() removes the top item and returns it.
• peek() returns the top item without removing it.
• is_empty() returns True when the stack holds nothing, and False otherwise.
• size() returns how many items the stack holds.

Calling pop() or peek() on an empty stack raises an IndexError.

Examples

s = Stack()
s.push('a')
s.push('b')
s.peek()       # returns 'b'
s.pop()        # returns 'b'
s.size()       # returns 1
s.is_empty()   # returns False
s.pop()        # returns 'a'
s.pop()        # raises IndexError

Note

Every stack keeps its own items: pushing onto one stack never changes another one.