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ACM today… ! (2009). DP for NP!. DP for APSP!. Max Flow. Knapsack. Floyd-Warshall. Harbin, China. 2010 finals. Jotto!. A word-guessing game similar to mastermind…. Sophs. Jrs. Srs. Profs. Chalk 1. Chalk 0. Chalk 1. Chalk 1. Quine 1. Quine 1. Quine 2. Quine 2. ?. ?. ?. ?.

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slide1

ACM today… ! (2009)

DP for NP!

DP for APSP!

Max Flow

Knapsack

Floyd-Warshall

Harbin, China

2010 finals

slide2

Jotto!

A word-guessing game similar to mastermind…

Sophs

Jrs

Srs

Profs

Chalk 1

Chalk 0

Chalk 1

Chalk 1

Quine 1

Quine 1

Quine 2

Quine 2

?

?

?

?

next guesses?

slide3

Candy!

1800\'s

1920\'s

1940\'s

1950\'s

1970\'s

slide4

v1 = 100

Maximizing Candy!

w1 = 2

i = 1

Suppose you can consume 13 candywt units of candy.

v2 = 120

w2 = 3

i = 2

What choice will maximize your candy-value experience?

v3 = 230

w3 = 5

i = 3

(1) if you can take fractional parts of candy packages…?

v4 = 560

w4 = 7

(2) if you can take any number of whole candy packages…?

i = 4

v5 = 675

(3) if you can take 0 or 1 of each whole candy package…?

w5 = 9

i = 5

slide5

v1 = 100

Maximizing Candy!

w1 = 2

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

v2 = 120

w2 = 3

i = 2

vpw2 = 40

What choice will maximize your candy-value experience?

v3 = 230

w3 = 5

GO WITH GREED!

i = 3

vpw3 = 46

(1) if you can take fractional parts of candy packages…?

v4 = 560

w4 = 7

(2) if you can take any number of whole candy packages…?

i = 4

vpw4 = 80

v5 = 675

(3) if you can take 0 or 1 of each whole candy package…?

w5 = 9

i = 5

vpw5 = 75

slide6

v1 = 100

Knapsack problem

w1 = 2

"unbounded"

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

v2 = 120

w2 = 3

(2) if you can take any number of whole candy packages…?

i = 2

vpw2 = 40

v3 = 230

w3 = 5

i = 3

vpw3 = 46

IDEA:

Consider all possible weights (integers) from 0 up to the weight you can carry -

For each one choose the best from all N items

v4 = 560

w4 = 7

i = 4

vpw4 = 80

v5 = 675

w5 = 9

i = 5

vpw5 = 75

slide7

v1 = 100

Knapsack problem

w1 = 2

"unbounded"

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

v2 = 120

w2 = 3

(2) if you can take any number of whole candy packages…?

i = 2

vpw2 = 40

v3 = 230

TOTAL WEIGHT

w3 = 5

0

1

2

3

4

5

6

7

8

9

10

11

12

13

i = 3

vpw3 = 46

0

0

v4 = 560

100

120

200

230

300

560

560

675

680

775

800

875

w4 = 7

i = 4

vpw4 = 80

max total value

v5 = 675

w5 = 9

max

V(w-wi) + vi

V(w) =

i

i = 5

vpw5 = 75

slide8

v1 = 100

Knapsack problem

w1 = 2

"0-1 problem"

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

v2 = 120

w2 = 3

(3) if you can take 0 or 1 of each whole candy package…?

i = 2

vpw2 = 40

v3 = 230

w3 = 5

i = 3

vpw3 = 46

IDEA:

v4 = 560

Do the same thing as before,

but consider sublists of items that grow one-at-a-time

w4 = 7

i = 4

vpw4 = 80

v5 = 675

w5 = 9

i = 5

vpw5 = 75

slide9

v1 = 100

Knapsack problem

w1 = 2

"0-1 problem"

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

(3) if you can take 0 or 1 of each whole candy package…?

v2 = 120

V(i-1,w)

w2 = 3

V(i,w) =

max

i = 2

vpw2 = 40

V(i,w-wi) + vi

i

v3 = 230

TOTAL WEIGHT

w3 = 5

0

1

2

3

4

5

6

7

8

9

10

11

12

13

i = 3

vpw3 = 46

0

01

v4 = 560

w4 = 7

012

i = 4

vpw4 = 80

0123

01234

v5 = 675

012345

w5 = 9

i = 5

Total Items

vpw5 = 75

max total value

slide10

elite

exact number of cows we need, indexed from 1 .. 100

Input

but the sum-of-squares of their indices must be < 30

3 30

Input

the number of different subsets of cows satisfying that constraint.

4

total

14

1 2 3

1 2 4

2 3 4

1 3 4

21

29

26

slide11

elite

exact number of cows we need, indexed from 1 .. 100

Input

but the sum-of-squares of their indices must be < 30

3 30

"use it or lose it"

Input

analysis

the number of different subsets of cows satisfying that constraint.

4

total

14

1 2 3

1 2 4

2 3 4

1 3 4

21

You\'re either going to USE the first cow - or you\'re not. Sum up both cases!

29

26

slide12

elite

exact number of cows we need, indexed from 1 .. 100

Input

but the sum-of-squares of their indices must be < 30

3 30

# of len-3 subsets, totaling under 30, starting from 1

N(3,30,1) =

N(2,29,2) + N(3,30,2)

Input

# of len-2 subsets, totaling under 29, starting from 2

# of len-3 subsets, totaling under 30, starting from 2

the number of different subsets of cows satisfying that constraint.

4

"use it or lose it"

total

14

1 2 3

1 2 4

2 3 4

1 3 4

21

29

26

slide14

elite

but it\'s much too slow!

slide17

Pythonfunction decorators

fib = dec42(fib)

implemented

slide18

DP for APSP…

all-pairs-shortest-paths

Directed graph as adjacency matrix:

dst

Directed graph:

"to"

1

2

3

4

100

inf

100

0

14

1

2

14

1

14

inf

0

50

2

50

src

"from"

inf

inf

0

14

3

10

14

inf

10

inf

0

4

4

3

14

0 intermediate nodes

slide19

Floyd-Warshall!

dst

dst

"to"

"to"

1

2

3

4

1

2

3

4

inf

100

inf

100

0

14

0

14

1

1

14

14

inf

0

50

inf

0

50

2

2

src

src

"from"

"from"

inf

inf

inf

0

14

inf

0

14

3

3

24

inf

10

inf

0

10

inf

0

4

4

1 intermediate node(s)!

0 intermediate nodes

1

slide20

Floyd-Warshall!

dst

dst

"to"

"to"

1

2

3

4

1

2

3

4

inf

64

inf

100

0

14

0

14

1

1

14

14

inf

0

50

inf

0

50

2

2

src

src

"from"

"from"

inf

inf

inf

0

14

inf

0

14

3

3

24

24

10

inf

0

10

inf

0

4

4

1 intermediate node(s)

2 intermediate node(s)!

1

1

2

slide21

Floyd-Warshall!

All-pairs shortest paths:

dst

dst

"to"

"to"

1

2

3

4

1

2

3

4

28

42

28

42

0

14

0

14

1

1

14

14

38

0

28

inf

0

28

2

2

src

src

"from"

"from"

24

inf

38

0

14

inf

0

14

3

3

24

24

10

38

0

10

38

0

4

4

3 intermediate node(s)

4 intermediate node(s) ~ done!

1

2

3

1

2

3

4

slide22

Floyd-Warshall!

dst

T[src][dst][k]

"to"

1

2

3

4

minimum distance from src to dst using intermediate nodes 1..k

28

42

0

14

1

T[src][dst][k-1]

inf

14

inf

0

28

2

min

=

src

T[src][k][k-1] +

T[k][dst][k-1]

14 + 10

"from"

inf

inf

0

14

3

24

10

38

0

4

24

4 intermediate node(s)

1

2

3

4

slide23

Tracking the paths…

Floyd Warshall…

not a huge amount of extra work to keep an extra table (pred) of the first intermediate vertex on the path from i to j

slide24

This week\'s problems…

These two are available if you haven\'t submitted them successfully before…

slide26

number of nodes

hurdles

Input

number of edges

number of "tasks"

5 6 3

1 2 12

3 2 8

1 3 5

2 5 3

3 4 4

2 4 8

3 4

1 2

5 1

from node 1 to node 2 the cost ("height") is 12

12

2

1

Output

3

8

5

What is the minimum cost along

any one of the edges required

to get from start to end ?

5

3

8

4

4

8

-1

4

not possible!

slide27

number of cows

dining

Input

total # of foods

total # of drinks

4 3 3

2 2 1 2 3 1

2 2 2 3 1 2

2 2 1 3 1 2

2 1 1 3 3

Likes

1 2

2 3

1 3

1 3

3 1

1 2

1 2

3

0

# of foods cow[i] likes

foods

drinks

1

# of drinks cow[i] likes

2

Output

3

What is a cow-satisfying assignment here?

3

# of cows that can receive both a food and a drink they like…

foods

drinks

each can be used only once

slide28

Max Flow

Ford-Fulkerson algorithm

12

B

D

20

16

sink

F

A

10

4

9

7

source

4

13

C

E

14

capacity

The problem how much traffic (flow) can get from the source to the sink ?

slide29

-

16

13

-

-

-

-

-

10

12

-

-

-

4

-

-

14

-

-

-

9

-

-

20

-

-

-

7

-

4

-

-

-

-

-

-

Max Flow

The problem how much traffic can get from the source to the sink ?

12

B

D

20

16

sink

F

A

10

4

9

7

source

4

13

“Capacity Graph”

C

E

14

TO

C

capacity

A

B

C

D

E

F

A

B

C

FROM

D

E

F

slide30

-

16

13

-

-

-

-

-

10

12

-

-

-

4

-

-

14

-

-

-

9

-

-

20

-

-

-

7

-

4

-

-

-

-

-

-

Find a path in C via BFS

The problem how much traffic can get from the source to the sink ?

Need to subtract this flow from C !

12

B

D

20

16

sink

F

A

10

4

9

7

source

4

13

C

E

14

TO

C

capacity

A

B

C

D

E

F

A

B

C

FROM

D

E

F

slide31

-

12

0

-

-

-

-12

-

0

12

-

-

-

0

-

-

0

-

-

-12

0

-

-

12

-

-

-

0

-

0

-

-

-

-12

-

-

Create F

Create a FLOW GRAPH with the minimum weight from that path

12

B

D

12

20

16

sink

12

12

F

A

10

4

9

7

source

4

13

“Flow Graph”

C

E

14

TO

F

capacity

A

B

C

D

E

F

A

B

flow in one direction is negative flow in the other direction

C

FROM

D

E

F

slide32

-

4

13

-

-

-

12

-

10

0

-

-

-

4

-

-

14

-

-

12

9

-

-

8

-

-

-

7

-

4

-

-

-

12

-

-

R = C - F

Use the RESIDUAL GRAPH with the rest of the capacity after that flow

0

B

D

12

8

4

sink

12

12

F

A

10

4

9

7

source

4

13

“Residual Graph”

C

E

14

TO

R

capacity

A

B

C

D

E

F

A

B

reverse edges allow the "undoing" of previous flow!

C

FROM

D

E

F

slide33

Keep going!

Continue running this on the residual capacity until BFS fails…

12/12

B

D

19/20

11/16

sink

F

A

0/10

1/4

0/9

7/7

source

4/4

12/13

C

E

11/14

max flow:23

Continue until there is no longer a path from A to F !

slide34

1. Set F to all 0

The max flow algorithm

2. Compute R = C - F

3. BFS in R from source to sink.

3a. If no path, you’re done.

3b. If  a path, add the available capacity to F; goto (2).

Floyd-Fulkerson

12

B

D

3

20

16

sink

2

4

1

0

F

A

10

4

2

9

1

7

2

source

4

4

13

C

5

E

14

capacity

flow

residual

“Residual Graph”

3

B

D

12

18

9

Don\'t need to keep R around explicity: Keep only the current flow (F) and the original capacity (C).

4

2

A

1

9

2

5

6

F

7

2

5

C

E

4

11

9

slide35

Get into the flow!

A little bit of name contention…

edmonds_karp

This is the algorithm.

defmax_flow(C, source, sink):

n = len(C) # C is the capacity matrix

F = [[0] * n for i in range(n)] # F is the flow matrix

# residual capacity from u to v is C[u][v] - F[u][v]

whileTrue:

path = BFS(C, F, source, sink)

if not path: break # no path - we\'re done!

# find the path\'s flow, that is, the "bottleneck"

edges = [C[u][v]-F[u][v] for u,v in path]

path_flow = min( edges )

print"Augmenting by", path_flow

for u,v in path: # traverse path to update flow

F[u][v] += path_flow # forward edge up

F[v][u] -= path_flow # backward edge down

return sum([F[source][i] for i inrange(n)]) # out from source

slide36

Useful alone, too

A brief BFS algorithm using the Capacity matrix

defBFS(C, F, source, sink):

queue = [source] # the BFS queue

paths = {source: []} # stores 1 path per graph node

while queue:

u = queue.pop(0) # next node to explore (expand)

for v inrange(len(C)): # for each possible next node

# path from u to v? and not yet at v?

if C[u][v] - F[u][v] > 0 and v notin paths:

paths[v] = paths[u] + [(u,v)]

if v == sink:

return paths[v]

queue.append(v) # go from v in the future

returnNone

slide37

Setting up…

And the code needed to run it…

if __name__ == "__main__":

# make a capacity graph

# node A B C D E F

C = [ [ 00, 16, 13, 00, 00, 00 ], # A

[ 00, 00, 10, 12, 00, 00 ], # B

[ 00, 04, 00, 00, 14, 00 ], # C

[ 00, 00, 9, 00, 00, 20 ], # D

[ 00, 00, 00, 7, 00, 4 ], # E

[ 00, 00, 00, 00, 00, 00 ] ] # F

print"C is", C

source = 0 # A

sink = 5 # F

max_flow_value = max_flow( C, source, sink )

print"max_flow_value is", max_flow_value

Linked at the ACM website by the slides…

slide38

Jotto!

A word-guessing game similar to mastermind…

Sophs

Jrs

Srs

Profs

Chalk 1

Chalk 0

Chalk 1

Chalk 1

Quine 1

Quine 1

Quine 2

Quine 2

?

?

?

?

next guesses?

slide39

elite eating

problem

how does the solution depend on subproblems?

use it or lose it!

slide40

v1 = 100

Knapsack problem

w1 = 2

"0-1 problem"

i = 1

vpw1 = 50

Suppose you can consume 13 candywt units of candy.

(3) if you can take 0 or 1 of each whole candy package…?

v2 = 120

w2 = 3

V(i,w) =

i = 2

vpw2 = 40

v3 = 230

TOTAL WEIGHT

w3 = 5

0

1

2

3

4

5

6

7

8

9

10

11

12

13

i = 3

vpw3 = 46

0

01

v4 = 560

w4 = 7

012

i = 4

vpw4 = 80

0123

01234

v5 = 675

012345

w5 = 9

i = 5

Total Items

vpw5 = 75

max total value

slide41

Candy!

1800\'s

1920\'s

1940\'s

1950\'s

1970\'s

ad