Roberto Selbach

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# Euler 15 in Python

This one isn’t even funny…

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?

Your first thought would be to generate the routes, but for a 20×20 grid, those amount to BILLIONS and you’d try to do it recursively too! Forget it.

But if you have some CompSci-level math background, though, you’ll remember this one—reading The Art of Computer Programming, Vol. 4 also helps. It’s a matter of combinatorics and if we take w for the width and h for the height, all we need to do is calculate,

$frac{(w + h)!}{w!h!}$

and that’s it. I didn’t even write a program for this, instead using Python’s interactive shell, but just for the same of completeness, here’s a “program” to calculate the possibles paths in a 20×20 grid.

``````import math
w = h = 20
print math.factorial(w + h) / (math.factorial(w) * math.factorial(h))
``````

That’s it.