[rosalind] add problem description to trie example

Author: paultcochrane

categories/rosalind/trie-grondilu.pl

@@ -6,15 +6,46 @@
 
 =AUTHOR L. Grondin
 
+=head2 Problem
+
+Given a collection of strings, their trie (often pronounced "try" to avoid
+ambiguity with the general term tree) is a rooted tree formed as follows.
+For every unique first symbol in the strings, an edge is formed connecting
+the root to a new vertex. This symbol is then used to label the edge.
+
+We may then iterate the process by moving down one level as follows. Say
+that an edge connecting the root to a node I<v> is labeled with 'A'; then we
+delete the first symbol from every string in the collection beginning with
+'A' and then treat v as our root. We apply this process to all nodes that
+are adjacent to the root, and then we move down another level and continue.
+See Figure 1 for an example of a trie.
+
+As a result of this method of construction, the symbols along the edges of
+any path in the trie from the root to a leaf will spell out a unique string
+from the collection, as long as no string is a prefix of another in the
+collection (this would cause the first string to be encoded as a path
+terminating at an internal node).
+
+Given: A list of at most 100 DNA strings of length at most 100 bp, none of
+which is a prefix of another.
+
+Return: The adjacency list corresponding to the trie I<T> for these
+patterns, in the following format. If I<T> has I<n> nodes, first label the
+root with 1 and then label the remaining nodes with the integers 2 through
+I<n> in any order you like. Each edge of the adjacency list of I<T> will be
+encoded by a triple containing the integer representing the edge's parent
+node, followed by the integer representing the edge's child node, and
+finally the symbol labeling the edge.
+
 L<http://rosalind.info/problems/trie/>
 
-Sample dataset:
+=head2 Sample dataset:
 
     ATAGA
     ATC
     GAT
 
-Sample output:
+=head2 Sample output:
 
     1 2 A
     2 3 T
@@ -26,7 +57,7 @@
     8 9 A
     9 10 T
 
-Usage:
+=head2 Usage:
 
     $ perl6 trie-grondilu.pl