Stack Operation      Stack Contents   Return Value
 s.isEmpty()   []        True
 s.push(4)   [4] 
 s.push('dog')   [4,'dog'] 
 s.peek()   [4,'dog']    'dog'
 s.push(True)   [4,'dog',True] 
 s.size()   [4,'dog',True]   3
 s.isEmpty()   [4,'dog',True]   False
 s.push(8.4)   [4,'dog',True,8.4] 
 s.pop()       [4,'dog',True]   8.4
 s.pop()       [4,'dog']     True
 s.size()   [4,'dog']     2

#coding:utf8
class Stack:
  """模拟栈"""
  def __init__(self):
    self.items = []
    
  def isEmpty(self):
    return len(self.items)==0 
  
  def push(self, item):
    self.items.append(item)
  
  def pop(self):
    return self.items.pop() 
  
  def peek(self):
    if not self.isEmpty():
      return self.items[len(self.items)-1]
    
  def size(self):
    return len(self.items) 
s=Stack()
print(s.isEmpty())
s.push(4)
s.push('dog')
print(s.peek())
s.push(True)
print(s.size())
print(s.isEmpty())
s.push(8.4)
print(s.pop())
print(s.pop())
print(s.size())