Lecture Notes on 21 Feb 2014

Problem: Flip pancakes until they are sorted with the 
largest at the bottom and the smallest at the top.

# finds the index of the local maximum
def findMaxIdx (top):
  size = len(top)
  idx = 0
  maxVal = top[0]
  for i in range (size):
    if (top[i] > maxVal):
      maxVal = top[i]
      idx = i
  return idx
  
# flips a sub-stack of pancakes twice so that the local
# maximum is at the bottom
def flip (a, idx):
  top = a[:idx]
  maxIdx = findMaxIdx (top)
  slice = a[:maxIdx + 1]
  slice.reverse()
  top = slice + top[(maxIdx + 1):]
  top.reverse()
  for i in range (idx):
    a[i] = top[i]
  
# Moves the largest pancake to the bottom iteratively
def pancake (a):
  size = len(a)
  for i in range (size, 0, -1):
    flip (a, i)

def main ():
  a = [9, 1, 8, 2, 7, 3, 6, 4, 5]
  print (a)
  pancake (a)
  print (a)

main()