Project Euler Problem 15 Solved

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Lattice paths

Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.

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

Solution

p15.py
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#!/usr/bin/python
# -*- coding: utf-8 -*-

if __name__ == '__main__':
    (steps, a, b) = (20, 1, 1)

    i = steps * 2
    while i > steps:
        a *= i
        i -= 1
    while steps > 1:
        b *= steps
        steps -= 1

    print a / b

I’m the 86747th person to have solved this problem.

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