[euler] add solution for problem 15 (just arithmetic, nothing cool here)

Author: felher

euler/prob015-felher.pl

@@ -0,0 +1,15 @@
+use v6;
+
+# This program doesn't really simulate anything and is only here for the sake of
+# completeness. But to show something cool about perl6 let's at least introduce
+# the faculty as the ! postfix operator
+my Int sub postfix:<!>(Int $n) { [*] 1 .. $n }
+
+# The strategy for this Problem is quite straight-forward: To move from the
+# upper left to the lower right corner of an NxN grid without ever going in the
+# wrong direction one has to go down N times and one has to go right N times.
+# The problem is therefore equivalent to "How many distinguishable ways exist to
+# order N white and N black balls?" and the answer to that problem is:
+
+my \N = 20;
+say (2 * N)!/(N! * N!);