Peek
by Isai Damier

#=====================================================================
# Author: Isai Damier
# Title: Stack With Get-Min
# Project: geekviewpoint
# Package: datastructure
#
# Description: Normally, only the top of a stack is available for
#    peeking and popping. Here, however, the requirement is to make
#    the minimum value on the stack always viewable in O(1).
#    To that end, in addition to the primary stack of operation
#    (called primary), a separate stack (called minimums) must be
#    used to track the minimum value on the primary stack. The point
#    is not to pop the minimum value in O(1) that would require a
#    bit more work. Rather, the getMinimum function allows peeking
#    into the stack, as it were, to see the minimum value on the
#    stack.
#
#    Throughout the implementation it should be clear that the
#    primary stack is the real (i.e. representative) stack and that
#    the minimums stack is an auxiliary stack that exists only so
#    the getMinimum() function may exist.
#=====================================================================
 
 from Queue import LifoQueue
class StackWithGetMin( object ):

  def __init__( self ):
      self.primary = LifoQueue()
      self.minimums = LifoQueue()
  #=====================================================================
  # Statement:
  #   Look at the element at the top of the stack without removing it
  #
  # Time Complexity of Solution:
  #   Best = Average = Worst = const.
  #
  # Technical Details: The self.primary stack is the real (i.e.
  #    representative) stack. The self.minimums stack exists only for the
  #    getMinimum function. If getMinimum were not important, all
  #    operations could as well be performed on the self.primary stack
  #    alone.
  #
  #=====================================================================
  def peek( self ):
    return self.primary.queue[self.primary.qsize() - 1]
import unittest
from algorithms.StackWithGetMin import StackWithGetMin

class StackWithGetMinTest( unittest.TestCase ):

  #=====================================================================
  # Test of peek method, of class StackWithGetMin.
  #=====================================================================

  def testPeek( self ):
    stack = StackWithGetMin()
    self.assertTrue( stack.isEmpty() )
    self.assertEquals( 0, stack.size() )
    inputs = [40, 65, 5, 6, 32, 4, 7, 1, 2, 84, 9, 10]
    for i in inputs:
      stack.push( i )

    self.assertFalse( stack.isEmpty() )
    self.assertEquals( len( inputs ), stack.size() )

    self.assertEquals( inputs[len( inputs ) - 1], stack.peek() )
    self.assertEquals( inputs[len( inputs ) - 1], stack.peek() )

    self.assertFalse( stack.isEmpty() )
    self.assertEquals( len( inputs ), stack.size() )