author: the openedg python institute

... stk = [ ] def push ( self , val ) : self .__ stk.append ( val ) def pop ( self ) : val = self .__ stk [ -1 ] del self .__ stk [ -1 ] return val 15 16 17 18 # 19 20 21 13 14 class CountingStack ( Stack ) : def __init __ ( self ) ...