Roman Forts

Time Limit: 2 Seconds
Memory Limit: 65536 KB

This program will acquaint you with the judicious method used by rulers of
the ancient Roman empire to choose the locations of their military forts.

You are given a collection of cities.

The cities will be labeled by upper case letters in alphabetical order, starting
with 'A'.

Given any pair of cities, there is one and only one path between them (see Figure
1).

This path may be a direct connection (as between J and C in Figure 1) or it
may pass through a number of intermediate cities (as between J and B in Figure
1).

In the example of Figure 1, the path from J to B passes through C, G, and F.

The actual number of miles between cities is irrelevant. The distance
between a pair of cities is defined as the number of hops (each "hop"
being taken over a over a direct connection) between them.

In Figure 1, the number of hops between J and C is 1, and the number of hops
between J and B is 4.

Initially, only one of the cities is fortified (by a military fort). As your
program executes, additional cities will be fortified.

If a city is not fortified, we say it is vulnerable .

Furthermore, if a city is vulnerable, its degree of vulnerability is defined
as the shortest distance between it and a fortified city.

When one or more cities have already been fortified, the location of an additional
fort is chosen by a simple rule: it is the most vulnerable city (the one having
the highest degree of vulnerability).

In case of a tie, choose the alphabetically first city among those tied as the
most vulnerable city.

Sample Input

11 J 5
A D
B E
F B
C G
F G
C A
G H
G I
J C
K F

The first line of the input contains three items of information,
separated by one or more blank spaces:

The total number of cities in the given collection. It will be in the range
3..26. In this example, it is 11, which means that the cities are labeled A..K.

An upper case letter in the valid range of cities, denoting the city which is
initially fortified.

The total number of cities that are to be fortified, including the initially
fortified one.

It will be at least three and not greater than the total number of cities in
the collection

Each of the remaining lines of the input file will contain two upper case letters,
separated by one or more blank spaces.

Each letter will be in the range of labels of cities in the given collection.

Each pair of letters on one line of input determines a direct connection between
two cities.

The direct connections listed in the input file shown on the right is identical
to the direct connections of Figure 1.

The set of direct connections listed in the input file will satisfy all previously
stated conditions.

All lines of input will be free of leading or trailing blank spaces.

The output will list the
cities to be fortified, as upper case letters, in the correct order.

Sample Output

Program 8 by team X
J E D H K
End of program 8 by team X

Explanation of the output:

When there is only one fortified city, the degree of vulnerability of each vulnerable
city is its distance from the fortified city.

Initially, with J being the only fortified city, E is the most vulnerable city
with degree of vulnerability 5.

With J and E being fortified, D, H, I, and K contend for being the most vulnerable
city, all having degree of vulnerability 3.

For example, the distance from D to E is 6, the distance from D to J is 3; the
smaller of these two numbers is 3.

Therefore, the degree of vulnerability of D is 3.

The alphabetically first city among the four contenders (D, H, I, and K) is
D, which will be the next city to be fortified.

Similarly, H, and then K, will be selected as the next city to be fortified.
At this point, the number of fortified cities is 5 (as specified in the input),
and the program terminates

Note that city I is not fortified, because its degree of vulnerability drops
from 3 to 2 once H is fortified, making K the most vulnerable city.

Formatting details:

The upper case letters denoting the cities in the output will be separated by
one blank space.

There will not be any blank lines or leading blank spaces in the output.

Source:

**Rocky Mountain 2000**
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