格路径

作者：Felix Herrmann

从一个 2×2 网格的左上角开始，每次只能向右或向下移动，则到达右下角恰好有 6 条路径。

那么在一个 20×20 的网格中，有多少条这样的路径？

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 factorial 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!);