Usaco Open11 Silver

Part of USACO Open11

                           SILVER PROBLEMS
                  Four problems numbered 6 through 9

Problem 6: Corn Maze [Sweet Tea Dorminy, 2010]

This past fall, Farmer John took the cows to visit a corn maze. But
this wasn't just any corn maze: it featured several gravity-powered
teleporter slides, which cause cows to teleport instantly from one
point in the maze to another. The slides work in both directions:
a cow can slide from the slide's start to the end instantly, or
from the end to the start. If a cow steps on a space that hosts
either end of a slide, she must use the slide.

The outside of the corn maze is entirely corn except for a single

The maze can be represented by an N x M (2 <= N <= 300; 2 <= M
<= 300) grid. Each grid element contains one of these items:

   * Corn (corn grid elements are impassable)

   * Grass (easy to pass through!)

   * A slide endpoint (which will transport a cow to the other

   * The exit

A cow can only move from one space to the next if they are adjacent
and neither contains corn. Each grassy space has four potential
neighbors to which a cow can travel. It takes 1 unit of time to
move from a grassy space to an adjacent space; it takes 0 units of
time to move from one slide endpoint to the other.

Corn-filled spaces are denoted with an octothorpe (#). Grassy spaces
are denoted with a period (.). Pairs of slide endpoints are denoted
with the same uppercase letter (A-Z), and no two different slides
have endpoints denoted with the same letter. The exit is denoted
with the equals sign (=).

Bessie got lost. She knows where she is on the grid, and marked her
current grassy space with the 'at' symbol (@). What is the minimum
time she needs to move to the exit space?

Consider the following grid, with N=5 and M=6:


A single slide has endpoints denoted by an uppercase W. Her optimal
strategy is to move right to the slide endpoint in 1 time, take the
slide in 0 time to the other endpoint, and move right and up in 2
more time to end on the exit.  This requires a total of 3 time, the

PROBLEM NAME: cornmaze


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

* Lines 2..N+1: Line i+1 describes row i of the maze: M characters (no


5 6


* Line 1: An integer, corresponding to the shortest time that Bessie
        needs to exit the maze.

SAMPLE OUTPUT (file cornmaze.out):



Problem 7: Cow Checkers [Amlesh Jayakumar, 2011]

One day, Bessie decides to challenge Farmer John to a game of 'Cow
Checkers'. The game is played on an M*N (1 <= M <= 1,000,000; 1 <=
N <= 1,000,000) checkerboard that initially contains a single checker
piece on the checkboard square coordinates (X, Y) (0 <= X < M; 0
<= Y < N). The bottom leftmost square of the checkerboard has
coordinates (0, 0), and the top rightmost square has coordinates
(M-1, N-1). Bessie always moves first, and then the two players
alternate turns.  Each turn comprises one of three types of moves:

1) Move the checker piece to any square on the same row to the left
   of its current position.

2) Move the checker piece to any square on the same column below its
   current position.

3) Move the checker piece to any spot k squares below and k squares
   to the left of the current square (where k is any positive integer
   such that this new spot is still on the checkerboard).

The first player unable to make a move (i.e., because the checker
is at (0, 0)) loses. Given that Bessie always goes first, determine
who will win the game if both play optimally.

Play and report the winner for T games (1 <= T <= 1,000) reading
in a new X,Y starting value for each new game.

PROBLEM NAME: cowcheck


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

* Line 2: A single integer: T

* Lines 3..T+2: Two space-separated integers: X and Y


3 3
1 1


Farmer John and Bessie are playing one game on a 3*3 checkerboard with the 
checker piece initially at (1, 1) (i.e. at the center of the board).


* Lines 1..T: Should contain either "Farmer John" or "Bessie"
        depending on who wins each  game.

SAMPLE OUTPUT (file cowcheck.out):



Bessie initially can only move the checker piece to either (1, 0) or (0, 
1), or (0, 0). Bessie can immediately win by moving the piece to (0, 0).


Problem 8: Forgotten Password [Damon Doucet, 2011]

As has happened to so many of us, Bessie has forgotten her cowtube
password. She does, however, remember some handy bits of information
about it.

First, she remembers that her password (denoted as variable P) has
length L (1 <= L <= 1,000) roman characters and can be split into
one or more words (not necessarily unique) from a dictionary composed
of NW (1 <= NW <= 1,000) unique words.  A word W_i is defined as a
sequence of 1..20 lowercase letters of the Roman alphabet ('a'..'z').

She also remembers certain letters from her password along with
their positions.

Consider the following example. Bessie knows that her password looks
like "a??l?ban???????" ('?' represents a letter she cannot remember),
and her dictionary has the following words:


The two possible passwords for Bessie to have are "applebananapies" and 

Given the dictionary, and the letters that Bessie remembers, please
find her password. If more than one password is valid, find the
lexicographically least string that works.



* Line 1: Two space-separated integers: L and NW

* Line 2: A string of length L: P

* Lines 3..NW+2: Line i+2 contains the ith word in the dictionary: W_i


15 6


* Line 1: The lexicographically least password that fits

SAMPLE OUTPUT (file forgot.out):



Problem 9: Learning Languages [Albert Gu, 2011]

Farmer John's N (2 <= N <= 10,000) cows, conveniently numbered 1..N,
are fluent in some M (1 <= M <= 30,000) languages, also conveniently
numbered from 1..M. Cow i can speak in K_i (1 <= K_i <= M) languages,
namely L_i1, L_i2,..., L_{iK_i} (1 <= L_ij <= M). FJ's cows aren't
THAT smart, so the sum of K_i over all cows i is at most 100,000.

Two cows can't directly talk to each other unless both speak a
common language. However, cows can pass messages along, translating
if necessary. In other words, cows A and B can have a conversation
if and only if there exists a sequence of cows T_1, T_2, ..., T_k
such that A and T_1 share a language, T_1 and T_2 share a language,
etc., and T_k and B share a language.

Farmer John wishes that his cows could be even more social, so he
wants all his cows to be able to socialize with any other cow. He
can buy books to teach any one of his cows any language he pleases.
Being a fairly frugal farmer, FJ wants to purchase the minimum
number of books necessary to enable all of his cows to speak to
each other. Help him determine:

    * The minimum number of books he must purchase

    * Any set of books assigned to cows in any order which will
      help him meet this goal; a program will grade your output.

By way of example, suppose there are three cows named Alberta,
Bessie, and Contessa along with three languages denoted as #1, #2,
and #3. Alberta can speak languages #2 and #3, Bessie can speak
language #2, and Contessa can speak language #1. Currently, Alberta
and Bessie can talk to each other, but Contessa is left alone.

                   #1   #2   #3
       Alberta           x    x
       Bessie            x
       Contessa     x

FJ wants to fix this situation, so he can buy Contessa a book to
teach her language #2. This will ensure all cows speak the same
language, so they can all communicate with one another.

Note that an alternate solution exists: instead, FJ could buy
Contessa a book to teach her language #3. Not all cows would speak
the same language, but this would still be a valid solution because
Contessa could communicate through Alberta (who also speaks language
#3) if she wants to talk to Bessie. Other alternatives exist, and any
valid alternate solution will also be accepted.



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

* Lines 2..N+1: Line i+1 describes the languages that cow i can speak
        with (K_i)+1 space-separated integers: K_i, L_i1,


3 3
2 3 2
1 2
1 1


* Line 1: A single integer that is the minimum number of books that FJ
        must purchase.

* Lines 2..B+1: Line i+1 contains two space-separated integers: the
        language id # and the id # of the cow to receive book i. If
        multiple solutions exist, print any one.

SAMPLE OUTPUT (file llang.out):

2 3

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