Usaco Jan09 Silver

Part of USACO Jan09

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SILVER PROBLEMS
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Three problems numbered 6 through 8
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Problem 6: Best Spot [Rob Kolstad, 2009]

Bessie, always wishing to optimize her life, has realized that she
really enjoys visiting F (1 <= F <= P) favorite pastures F_i of the
P (1 <= P <= 500; 1 <= F_i <= P) total pastures (conveniently
numbered 1..P) that compose Farmer John's holdings.

Bessie knows that she can navigate the C (1 <= C <= 8,000) bidirectional
cowpaths (conveniently numbered 1..C) that connect various pastures
to travel to any pasture on the entire farm. Associated with each
path P_i is a time T_i (1 <= T_i <= 892) to traverse that path (in
either direction) and two path endpoints a_i and b_i (1 <= a_i <=
P; 1 <= b_i <= P).

Bessie wants to find the number of the best pasture to sleep in so
that when she awakes, the average time to travel to any of her F
favorite pastures is minimized.

By way of example, consider a farm laid out as the map below shows,
where *'d pasture numbers are favorites. The bracketed numbers are
times to traverse the cowpaths.

1*--[4]--2--[2]--3
|       |
[3]     [4]
|       |
4--[3]--5--[1]---6---[6]---7--[7]--8*
|       |        |         |
[3]     [2]      [1]       [3]
|       |        |         |
13*      9--[3]--10*--[1]--11*--[3]--12*

The following table shows distances for potential 'best place' of
pastures 4, 5, 6, 7, 9, 10, 11, and 12:

* * * * * * Favorites * * * * * *
Potential      Pasture Pasture Pasture Pasture Pasture Pasture     Average
Best Pasture       1       8      10      11      12      13        Distance
------------      --      --      --      --      --      --      -----------
4              7      16       5       6       9       3      46/6 = 7.67
5             10      13       2       3       6       6      40/6 = 6.67
6             11      12       1       2       5       7      38/6 = 6.33
7             16       7       4       3       6      12      48/6 = 8.00
9             12      14       3       4       7       8      48/6 = 8.00
10             12      11       0       1       4       8      36/6 = 6.00 ** BEST
11             13      10       1       0       3       9      36/6 = 6.00
12             16      13       4       3       0      12      48/6 = 8.00

Thus, presuming these choices were the best ones (a program would
have to check all of them somehow), the best place to sleep is
pasture 10.

PROBLEM NAME: bestspot

INPUT FORMAT:

* Line 1: Three space-separated integers: P, F, and C

* Lines 2..F+1: Line i+2 contains a single integer: F_i

* Lines F+2..C+F+1: Line i+F+1 describes cowpath i with three
space-separated integers: a_i, b_i, and T_i

SAMPLE INPUT (file bestspot.in):

13 6 15
11
13
10
12
8
1
2 4 3
7 11 3
10 11 1
4 13 3
9 10 3
2 3 2
3 5 4
5 9 2
6 7 6
5 6 1
1 2 4
4 5 3
11 12 3
6 10 1
7 8 7

INPUT DETAILS:

As the problem statement

OUTPUT FORMAT:

* Line 1: A single line with a single integer that is the best pasture
in which to sleep. If more than one pasture is best, choose
the smallest one.

SAMPLE OUTPUT (file bestspot.out):

10

OUTPUT DETAILS:

As the problem statement.

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Problem 7: Total Flow [Rob Kolstad, 2008]

Farmer John always wants his cows to have enough water and thus has
made a map of the N (1 <= N <= 700) water pipes on the farm that
connect the well to the barn. He was surprised to find a wild mess
of different size pipes connected in an apparently haphazard way.
He wants to calculate the flow through the pipes.

Two pipes connected in a row allow water flow that is the minimum
of the values of the two pipe's flow values. The example of a pipe
with flow capacity 5 connecting to a pipe of flow capacity 3 can
be reduced logically to a single pipe of flow capacity 3:

+---5---+---3---+    ->    +---3---+

Similarly, pipes in parallel let through water that is the sum of
their flow capacities:

+---5---+
---+       +---    ->    +---8---+
+---3---+

Finally, a pipe that connects to nothing else can be removed; it
contributes no flow to the final overall capacity:

+---5---+
---+               ->    +---3---+
+---3---+--

All the pipes in the many mazes of plumbing can be reduced using
these ideas into a single total flow capacity.

Given a map of the pipes, determine the flow capacity between the
well (A) and the barn (Z).

Consider this example where node names are labeled with letters:

+-----------6-----------+
A+---3---+B                      +Z
+---3---+---5---+---4---+
C       D

Pipe BC and CD can be combined:

+-----------6-----------+
A+---3---+B                      +Z
+-----3-----+-----4-----+
D

Then BD and DZ can be combined:

+-----------6-----------+
A+---3---+B                      +Z
+-----------3-----------+

Then two legs of BZ can be combined:

B
A+---3---+---9---+Z

Then AB and BZ can be combined to yield a net capacity of 3:

A+---3---+Z

Write a program to read in a set of pipes described as two endpoints
and then calculate the net flow capacity from 'A' to 'Z'. All
networks in the test data can be reduced using the rules here.

Pipe i connects two different nodes a_i and b_i (a_i in range
'A-Za-z'; b_i in range 'A-Za-z') and has flow F_i (1 <= F_i <=
1,000). Note that lower- and upper-case node names are intended
to be treated as different.

The system will provide extra test case feedback for your first 50
submissions.

PROBLEM NAME: flow

INPUT FORMAT:

* Line 1: A single integer: N

* Lines 2..N + 1: Line i+1 describes pipe i with two letters and an
integer, all space-separated: a_i, b_i, and F_i

SAMPLE INPUT (file flow.in):

5
A B 3
B C 3
C D 5
D Z 4
B Z 6

OUTPUT FORMAT:

* Line 1: A single integer that the maximum flow from the well ('A')
to the barn ('Z')

SAMPLE OUTPUT (file flow.out):

3

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Problem 8: Laserphones [Rob Kolstad, 2008]

The cows have a new laser-based system so they can have casual
conversations while out in the pasture which is modeled as a W x H
grid of points (1 <= W <= 100; 1 <= H <= 100).

The system requires a sort of line-of-sight connectivity in order
to sustain communication. The pasture, of course, has rocks and
trees that disrupt the communication but the cows have purchased
diagonal mirrors ('/' and '\' below) that deflect the laser beam
through a 90 degree turn. Below is a map that illustrates the
problem.

H is 8 and W is 7 for this map.  The two communicating cows are
notated as 'C's; rocks and other blocking elements are notated as
'*'s:

7 . . . . . . .         7 . . . . . . .
6 . . . . . . C         6 . . . . . /-C
5 . . . . . . *         5 . . . . . | *
4 * * * * * . *         4 * * * * * | *
3 . . . . * . .         3 . . . . * | .
2 . . . . * . .         2 . . . . * | .
1 . C . . * . .         1 . C . . * | .
0 . . . . . . .         0 . \-------/ .
0 1 2 3 4 5 6           0 1 2 3 4 5 6

Determine the minimum number of mirrors M that must be installed
to maintain laser communication between the two cows, a feat which
is always possible in the given test data.

PROBLEM NAME: lphone

INPUT FORMAT:

* Line 1: Two space separated integers: W and H

* Lines 2..H+1: The entire pasture.

SAMPLE INPUT (file lphone.in):

7 8
.......
......C
......*
*****.*
....*..
....*..
.C..*..
.......

OUTPUT FORMAT:

* Line 1: A single integer: M

SAMPLE OUTPUT (file lphone.out):

3

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