Problem 15

Starting in the top left corner of a 2×2 grid, there are 6 routes (without backtracking) to the bottom right corner.

How many routes are there through a 20×20 grid?

Integerを使えば簡単。

p015 n = product[n+1..2*n]`div`product[1..n]
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Newer// Problem 14
Older// Problem 16