Usaco Jan11 Gold

Part of USACO Jan11

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GOLD PROBLEMS
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Three problems numbered 1 through 3
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Problem 1: Bottleneck [John Pardon, 2010]

Farmer John is gathering the cows. His farm contains a network of
N (1 <= N <= 100,000) fields conveniently numbered 1..N and connected
by N-1 unidirectional paths that eventually lead to field 1. The
fields and paths form a tree.

Each field i > 1 has a single one-way, exiting path to field P_i,
and currently contains C_i cows (1 <= C_i <= 1,000,000,000). In
each time unit, no more than M_i (0 <= M_i <= 1,000,000,000) cows
can travel from field i to field P_i (1 <= P_i <= N) (i.e., only
M_i cows can traverse the path).

Farmer John wants all the cows to congregate in field 1 (which has
no limit on the number of cows it may have). Rules are as follows:

* Time is considered in discrete units.

* Any given cow might traverse multiple paths in the same time
unit. However, no more than M_i total cows can leave field i
(i.e., traverse its exit path) in the same time unit.

* Cows never move *away* from field #1.

In other words, every time step, each cow has the choice either to

a) stay in its current field

b) move through one or more fields toward field #1, as long
as the bottleneck constraints for each path are not violated

Farmer John wants to know how many cows can arrive in field 1 by
certain times. In particular, he has a list of K (1 <= K <= 10,000)
times T_i (1 <= T_i <= 1,000,000,000), and he wants to know, for
each T_i in the list, the maximum number of cows that can arrive
at field 1 by T_i if scheduled to optimize this quantity.

Consider an example where the tree is a straight line, and the T_i
list contains only T_1=5, and cows are distibuted as shown:

Locn:      1---2---3---4      <-- Pasture ID numbers
C_i:       0   1   12  12     <-- Current number of cows
M_i:           5   8   3      <-- Limits on path traversal; field
1 has no limit since it has no exit

The solution is as follows; the goal is to move cows to field 1:

Tree:      1---2---3---4
t=0        0   1   12  12     <-- Initial state
t=1        5   4   7   9      <-- field 1 has cows from field 2 and 3
t=2        10  7   2   6
t=3        15  7   0   3
t=4        20  5   0   0
t=5        25  0   0   0

Thus, the answer is 25: all 25 cows can arrive at field 1 by time
t=5.

PROBLEM NAME: bottleneck

INPUT FORMAT:

* Line 1: Two space-separated integers: N and K

* Lines 2..N: Line i (not i+1) describes field i with three
space-separated integers: P_i, C_i, and M_i

* Lines N+1..N+K: Line N+i contains a single integer: T_i

SAMPLE INPUT (file bottleneck.in):

4 1
1 1 5
2 12 7
3 12 3
5

OUTPUT FORMAT:

* Lines 1..K: Line i contains a single integer that is the maximum
number of cows that can arrive at field 1 by time T_i.

SAMPLE OUTPUT (file bottleneck.out):

25

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Problem 2: The Continental Cowngress [Louis Wasserman, 2010]

from the farm and have formed the first Continental Cowngress.
Built on the principle of "every cow gets something they want,"
they've decided on the following voting system:

The M (1 <= M <= 4000) cows in attendance will vote on  N (1 <=
N <= 1,000) legislative bills. Each cow casts a "yes" or "no"
vote (denoted as 'Y' or 'N' in the input file) on exactly two
(distinct) bills B_i and C_i (1 <= B_i <= N; 1 <= C_i <= N). The
votes are called VB_i (VB_i in {'Y', 'N'}) and VC_i (VC_i in
{'Y', 'N'}) respectively.

Finally, the bills are to be passed or not in such a way that
every cow gets her way on at least one of her votes. For example,
if Bessie votes "yes" on Bill 1, and "no" on Bill 2, then in any
valid solution, it must be the case that either Bill 1 gets
passed or Bill 2 gets rejected (or both).

Given the votes of each of the cows, it's your job to figure out
which bills will be passed and which bills will be rejected in order
to conform to the rules above.  If there is no solution, print
"IMPOSSIBLE". If there is at least one solution, then for each bill,
display:

Y  if in every solution this bill passes
N  if in every solution this bill fails
?  if there are solutions where this bill passes and solutions
where it does not pass

Consider the following set of votes (two for each cow):

- - - - - BILL - - - - -
1        2        3
Cow 1   YES      NO
Cow 2   NO       NO
Cow 3   YES               YES
Cow 4   YES      YES

From this, two solutions satisfy every cow:
* Bill 1 passes (this then satisfies cows 1, 3, and 4)
* Bill 2 fails (this then satisfies cow 2)
* Bill 3 could pass or fail (and this is the reason there are
two solutions)

In fact, these are the only two solutions, so the answer is the
three character string below:

YN?

PROBLEM NAME: cowngress

INPUT FORMAT:

* Line 1: Two space-separated integers: N and M

* Lines 2..M+1: Line i+1 describes cow i's votes with four
space-separated fields -- an integer, a vote, another integer,
and another vote: B_i, VB_i, C_i, VC_i

SAMPLE INPUT (file cowngress.in):

3 4
1 Y 2 N
1 N 2 N
1 Y 3 Y
1 Y 2 Y

OUTPUT FORMAT:

* Line 1: A string with N characters, where the ith character is
either a "Y" if the ith bill must pass, an "N" if the ith bill
must fail, or a "?" if it cannot be determined whether the

If there is no solution which satisfies every cow, then output the
single line "IMPOSSIBLE".

SAMPLE OUTPUT (file cowngress.out):

YN?

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Problem 3: Roads and Planes [Michael Cohen, 2010]

Farmer John is conducting research for a new milk contract in a new
territory. He intends to distribute milk to T (1 <= T <= 25,000)
towns conveniently numbered 1..T which are connected by up to R (1
<= R <= 50,000) roads conveniently numbered 1..R and/or P (1 <= P
<= 50,000) airplane flights conveniently numbered 1..P.

Either road i or plane i connects town A_i (1 <= A_i <= T) to town
B_i (1 <= B_i <= T) with traversal cost C_i. For roads, 0 <= C_i
<= 10,000; however, due to the strange finances of the airlines,
the cost for planes can be quite negative (-10,000 <= C_i <= 10,000).

Roads are bidirectional and can be traversed from A_i to B_i or B_i
to A_i for the same cost; the cost of a road must be non-negative.

Planes, however, can only be used in the direction from A_i to B_i
specified in the input. In fact, if there is a plane from A_i to
B_i it is guaranteed that there is no way to return from B_i to A_i
with any sequence of roads and planes due to recent antiterror
regulation.

Farmer John is known around the world as the source of the world's
finest dairy cows. He has in fact received orders for his cows from
every single town. He therefore wants to find the cheapest price
for delivery to each town from his distribution center in town S
(1 <= S <= T) or to know that it is not possible if this is the
case.

MEMORY LIMIT: 64MB

INPUT FORMAT:

* Line 1: Four space separated integers: T, R, P, and S

* Lines 2..R+1: Three space separated integers describing a road: A_i,
B_i and C_i

* Lines R+2..R+P+1: Three space separated integers describing a plane:
A_i, B_i and C_i

6 3 3 4
1 2 5
3 4 5
5 6 10
3 5 -100
4 6 -100
1 3 -10

INPUT DETAILS:

6 towns.  There are roads between town 1 and town 2, town 3 and town 4, and
town 5 and town 6 with costs 5, 5 and 10; there are planes from town 3 to
town 5, from town 4 to town 6, and from town 1 to town 3 with costs -100, -
100 and -10.  FJ is based in town 4.

OUTPUT FORMAT:

* Lines 1..T: The minimum cost to get from town S to town i, or 'NO
PATH' if this is not possible

NO PATH
NO PATH
5
0
-95
-100

OUTPUT DETAILS:

FJ's cows begin at town 4, and can get to town 3 on the road.  They
can get to towns 5 and 6 using planes from towns 3 and 4.  However,
there is no way to get to towns 1 and 2, since they cannot go
backwards on the plane from 1 to 3.

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page revision: 2, last edited: 08 Jul 2011 02:33