Time Limit: 10 Seconds
Memory Limit: 32768 KB
A relatively simple method for compressing data works by creating a so-called
Huffman tree for a file and using it to compress and decompress the data it
contains. For most applications, binary Huffman trees are used (i.e., each node
is either a leaf or has exactly two sub-nodes). One can, however, construct
Huffman trees with an arbitrary number of sub-trees (i.e, ternary or, in general,
A Huffman tree for a file containing Z different characters has Z leaves. The
path from the root to a leaf that represents a certain character determines
its encoding; each step towards the leaf determines an encoding character (which
can be 0, 1, ..., (N-1)). By placing often-occurring characters closer to the
root, and less often occurring characters further away from the root, the desirable
compression is achieved. Strictly speaking, such a tree is a Huffman tree only
if the resulting encoding takes the minimal number of N-ary symbols to encode
the complete file.
For this problem, we only consider trees where each node is either an internal
node, or a leaf encoding a character; there are no dangling leaves that do not
encode for a character.
The figure below shows a sample ternary Huffman tree; the characters 'a' and
'e' are encoded using one ternary symbol; the less frequently occurring characters
's' and 'p' are encoded using two ternary symbols; and the very rare symbols
'x', 'q' and 'y' are encoded using three ternary symbols each.
Of course, if we want to decompress a list of N-ary symbols later on, it is
important to know which tree was used to compress the data. This can be done
in several ways. In this problem we use the following method: the stream of
data will be preceded by a header consisting of the encoded versions of the
Z characters occurring in the original file, in lexicographical order.
Given the number of source symbols Z, a value N denoting the 'arity' of the
Huffman tree, and a header, give the mapping from source symbols to encoded
The input starts with a single integer T on a separate line, denoting the number
of test cases that follow. Next, for each of the T test cases, three lines follow:
> The number of different characters in the file (2 <= Z <= 20);
> The number N, denoting the 'arity' of the Huffman tree (2 <= N <=
> A string representing the header of the received message; each character
will be a digit in the range [0, (N-1)]. This string will contain less than
Note: Although rare, it is possible for a header to have multiple interpretations
(e.g., the header '010011101100' with Z = 5 and N = 2). It is guaranteed that
all cases in the test input have a unique solution.
For each of the T test-cases, print Z lines giving the encoded version of each
of the Z characters in the testcase in ascending order. Use the format original->encoding,
where original is a decimal value in the range [0, (Z-1)] and encoding is the
string of encoding digits for this symbol (each digit is >= 0 and < N).
Source: Northwestern Europe 2002