factor/extra/project-euler/015/015.factor

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Factor

! Copyright (c) 2007 Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel math math.combinatorics project-euler.common ;
IN: project-euler.015
! http://projecteuler.net/index.php?section=problems&id=15
! DESCRIPTION
! -----------
! Starting in the top left corner of a 2x2 grid, there are 6 routes (without
! backtracking) to the bottom right corner.
! How many routes are there through a 20x20 grid?
! SOLUTION
! --------
<PRIVATE
: grid-paths ( n -- n )
dup 2 * swap nCk ;
PRIVATE>
: euler015 ( -- answer )
20 grid-paths ;
! [ euler015 ] 100 ave-time
! 0 ms ave run time - 0.2 SD (100 trials)
SOLUTION: euler015