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Pancakes With A Problem!

Pancakes With A Problem!. Course Staff. Prof: Bruce Maggs (Carnegie Mellon) TA: Souvik Sen Lectures developed at Carnegie Mellon, primarily by Prof. Steven Rudich. ((( ))). Please feel free to ask questions!. Course Document You must read this carefully.

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Pancakes With A Problem!

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  1. Pancakes With A Problem!

  2. Course Staff • Prof: Bruce Maggs (Carnegie Mellon) • TA: Souvik Sen • Lectures developed at Carnegie Mellon, primarily by Prof. Steven Rudich.

  3. ((( ))) Please feel free to ask questions!

  4. Course DocumentYou must read this carefully. • Grading formula for the course. • 40% homework • 30% quizzes • 30% final • Seven points a day late penalty. • Collaboration/Cheating Policy • You may NOT share written work. • We reuse homework problems.

  5. The chef at our place is sloppy, and when he prepares a stack of pancakes they come out all different sizes. Therefore, when I deliver them to a customer, on the way to the table I rearrange them (so that the smallest winds up on top, and so on, down to the largest at the bottom). I do this by grabbing several from the top and flipping them over, repeating this (varying the number I flip) as many times as necessary.

  6. Developing A Notation:Turning pancakes into numbers

  7. 5 2 3 4 1 Developing A Notation:Turning pancakes into numbers

  8. Developing A Notation:Turning pancakes into numbers 5 2 3 4 1

  9. Developing A Notation:Turning pancakes into numbers 52341

  10. How do we sort this stack?How many flips do we need? 52341

  11. 4 Flips Are Sufficient 52341 14325 23415 43215 12345

  12. Algebraic Representation 52341 X = The smallest number of flips required to sort: Upper Bound ?  X  ? Lower Bound

  13. Algebraic Representation 52341 X = The smallest number of flips required to sort: Upper Bound ?  X  4 Lower Bound

  14. 4 Flips Are Necessary 52341 14325 41325 If we could do it in 3 flips Flip 1 has to put 5 on bottom Flip 2 must bring 4 to top (if it didn’t we’d need more than 3 flips).

  15. ?  X  4 Lower Bound

  16. X = 4 4  X  4 Lower Bound Upper Bound

  17. 5th Pancake Number P5 = The number of flips required to sort the worst case stack of 5 pancakes. Upper Bound ?  P5 ? Lower Bound

  18. 5th Pancake Number P5 = The number of flips required to sort the worst case stack of 5 pancakes. Upper Bound 4  P5 ? Lower Bound

  19. The 5th Pancake Number: The MAX of the X’s 1 2 3 52341 119 120 . . . . . . . X120 X1 4 X2 X3 X119

  20. P5 = MAX over s 2 stacks of 5 of MIN # of flips to sort s 1 2 3 52341 199 120 . . . . . . . X1 4 X2 X3 X119 X120

  21. Pn MAX over s  stacks of n pancakes of MIN # of flips to sort s Or, The number of flips required to sort a worst-case stack of n pancakes.

  22. Pn = The number of flips required to sort a worst-case stack of n pancakes. Be Cool. Learn Math-speak.

  23. What is Pn for small n? Can you do n = 0, 1, 2, 3 ?

  24. Initial Values Of Pn

  25. P3 = 3 • 132 requires 3 Flips, hence P3¸ 3. • ANY stack of 3 can be done in 3 flips. • Get the big one to the bottom (· 2 flips). • Use · 1 more flip to handle the top two. • Hence, P3· 3.

  26. nth Pancake Number Pn = Number of flips required to sort a worst case stack of n pancakes. Upper Bound ?  Pn ? Lower Bound

  27. ?  Pn ? Take a few minutes to try and prove bounds on Pn,for n>3.

  28. Bring To Top Method Bring biggest to top. Place it on bottom. Bring next largest to top. Place second from bottom. And so on…

  29. Upper Bound On Pn:Bring To Top Method For n Pancakes • If n=1, no work - we are done. • Else: flip pancake n to top and then flip it to position n. • Now use: Bring To Top Method For n-1 Pancakes Total Cost: at most 2(n-1) = 2n –2 flips.

  30. Better Upper Bound On Pn:Bring To Top Method For n Pancakes • If n=2, use one flip and we are done. • Else: flip pancake n to top and then flip it to position n. • Now use: Bring To Top Method For n-1 Pancakes Total Cost: at most 2(n-2) + 1 = 2n –3 flips.

  31. Bring to top not always optimal for a particular stack • 5 flips, but can be done in 4 flips 52341 14325 41325 23145 32145

  32. ?  Pn 2n - 3 What bounds can you prove on Pn?

  33. Breaking Apart Argument 9 16 • Suppose a stack S contains a pair of adjacent pancakes that will not be adjacent in the sorted stack. • Any sequence of flips that sorts stack S must involve one flip that inserts the spatula between that pair and breaks them apart.

  34. Breaking Apart Argument 916 • Suppose a stack S contains a pair of adjacent pancakes that will not be adjacent in the sorted stack. • Any sequence of flips that sorts stack S must involve one flip that inserts the spatula between that pair and breaks them apart. • Furthermore, this same principle is true of the “pair” formed by the bottom pancake of S and the plate.

  35. n  Pn S 2468..n1357..n-1 • Suppose n is even. • Such a stack S contains n pairs that must be broken apart during any sequence that sorts stack S.

  36. n  Pn S 21 • Suppose n is even. • Such a stack S contains n pairs that must be broken apart during any sequence that sorts stack S. Detail: This construction only works when n>2

  37. n  Pn S 1357..n 2468..n-1 • Suppose n is odd. • Such a stack S contains n pairs that must be broken apart during any sequence that sorts stack S.

  38. n  Pn S 132 • Suppose n is odd. • Such a stack S contains n pairs that must be broken apart during any sequence that sorts stack S. Detail: This construction only works when n>3

  39. n ≤ Pn≤ 2n – 3 (for n ≥ 3) Bring To Top is within a factor of two of optimal!

  40. n ≤ Pn≤ 2n – 3 (for n ≥ 3) So starting from ANY stack we can get to the sorted stack using no more than Pn flips.

  41. ((( ))) From ANY stack to sorted stack in · Pn. From sorted stack to ANY stack in · Pn ? Reverse the sequences we use to sort.

  42. From ANY stack to sorted stack in · Pn. From sorted stack to ANY stack in · Pn. Hence,From ANY stack to ANY stack in · 2Pn.

  43. ((( ))) From ANY stack to ANY stack in · 2Pn. Can you find a faster way than 2Pn flips to go from ANY to ANY?

  44. S:4,3,5,1,2 T:5,2,4,3,1 From ANY Stack S to ANY stack T in · Pn Rename the pancakes in S to be 1,2,3,..,n. Rewrite T using the new naming scheme that you used for S. T will be some list: p(1),p(2),..,p(n). The sequence of flips that brings the sorted stack to p(1),p(2),..,p(n) will bring S to T. 1,2,3,4,5 3,5,1,2,4

  45. 12345678910111213 01345 The Known Pancake Numbers Pn n 7891011131415

  46. P14 Is Unknown • 14! Orderings of 14 pancakes. • 14! = 87,178,291,200

  47. Is This Really Computer Science?

  48. Posed in Amer. Math. Monthly 82 (1) (1975), “Harry Dweighter” a.k.a. Jacob Goodman

  49. (17/16)n  Pn (5n+5)/3 Bill Gates & Christos Papadimitriou: Bounds For Sorting By Prefix Reversal. Discrete Mathematics, vol 27, pp 47-57, 1979.

  50. (15/14)n  Pn (5n+5)/3 H. Heydari & Ivan H. Sudborough. On the Diameter of the Pancake Network. Journal of Algorithms, vol 25, pp 67-94, 1997.

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