## Google FooBar – Level 2 – Gearing up for Destruction – Python

I’ve recently been learning Python and was invited to the Google Foobar challenge. Google Foobar is an invitation only event that appears if Google detects that you’ve been searching for Python or Java related topics. You can accept the even and complete 5 levels which if you complete you will be able to fill out your personal information for a chance at an interview at Google. My skills aren’t the best but I had some trouble figuring out some of the levels so I decided to publish my submissions in hopefully helping someone out.

### Objective

To complete the Google Foobar Level 2 challenge, Gearing up for Destruction

### Instructions

I found this puzzle very difficult since I did not understand much about gear ratio’s and the formulas associated with them. Your best bet is to read about Gear Trains on the Wikipedia page and follow along some of the formulas there and understand the concept of Gear Ratio’s.

### The Challenge

As Commander Lambda’s personal assistant, you’ve been assigned the task of configuring the LAMBCHOP doomsday device’s axial orientation gears. It should be pretty simple – just add gears to create the appropriate rotation ratio. But the problem is, due to the layout of the LAMBCHOP and the complicated system of beams and pipes supporting it, the pegs that will support the gears are fixed in place.

The LAMBCHOP’s engineers have given you lists identifying the placement of groups of pegs along various support beams. You need to place a gear on each peg (otherwise the gears will collide with unoccupied pegs). The engineers have plenty of gears in all different sizes stocked up, so you can choose gears of any size, from a radius of 1 on up. Your goal is to build a system where the last gear rotates at twice the rate (in revolutions per minute, or rpm) of the first gear, no matter the direction. Each gear (except the last) touches and turns the gear on the next peg to the right.

Given a list of distinct positive integers named pegs representing the location of each peg along the support beam, write a function answer(pegs) which, if there is a solution, returns a list of two positive integers a and b representing the numerator and denominator of the first gear’s radius in its simplest form in order to achieve the goal above, such that radius = a/b. The ratio a/b should be greater than or equal to 1. Not all support configurations will necessarily be capable of creating the proper rotation ratio, so if the task is impossible, the function answer(pegs) should return the list [-1, -1].

For example, if the pegs are placed at [4, 30, 50], then the first gear could have a radius of 12, the second gear could have a radius of 14, and the last one a radius of 6. Thus, the last gear would rotate twice as fast as the first one. In this case, pegs would be [4, 30, 50] and answer(pegs) should return [12, 1].

The list pegs will be given sorted in ascending order and will contain at least 2 and no more than 20 distinct positive integers, all between 1 and 10000 inclusive.

### Test cases

1 2 3 4 5 6 7 8 9 | Inputs: (int list) pegs = [4, 30, 50] Output: (int list) [12, 1] Inputs: (int list) pegs = [4, 17, 50] Output: (int list) [-1, -1] |

### The Solution

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | from fractions import Fraction def answer(pegs): arrLength = len(pegs) if ((not pegs) or arrLength == 1): return [-1,-1] even = True if (arrLength % 2 == 0) else False sum = (- pegs[0] + pegs[arrLength - 1]) if even else (- pegs[0] - pegs[arrLength -1]) if (arrLength > 2): for index in xrange(1, arrLength-1): sum += 2 * (-1)**(index+1) * pegs[index] FirstGearRadius = Fraction(2 * (float(sum)/3 if even else sum)).limit_denominator() currentRadius = FirstGearRadius for index in xrange(0, arrLength-2): CenterDistance = pegs[index+1] - pegs[index] NextRadius = CenterDistance - currentRadius if (currentRadius < 1 or NextRadius < 1): return [-1,-1] else: currentRadius = NextRadius return [FirstGearRadius.numerator, FirstGearRadius.denominator] |