#=====================================================================
# 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:
# Add the given element to the stack.
#
# Time Complexity of Solution:
# Best = Average = Worst = const.
#
# Technical Details: If the stack is empty, then the given element
# is perforce also the minimum element on the stack. Therefore
# mark the element as minimum by also pushing it into the
# minimums stack. On the other hand, if the stack (i.e. the
# primary stack) is not empty, check if the new element is less
# than the current minimum element. If it is, make the new
# element the new minimum.
#
# The explanation is slightly different from the actual code
# to give the reader a different viewpoint.
#
#=====================================================================
def push( self, e ):
if self.primary.empty() or e < self.minimums.queue[self.minimums.qsize() - 1]:
self.minimums.put( e )
self.primary.put( e )
import unittest
from algorithms.StackWithGetMin import StackWithGetMin
class StackWithGetMinTest( unittest.TestCase ):
#=====================================================================
# Test of push method, of class StackWithGetMin.
#=====================================================================
def testPush( self ):
e = 0
stack = StackWithGetMin()
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() )