Part 1

We can imagine the dial as a circular clock which is numbered from 0 to 99 and we start at 50.

Every rotation instruction (L or R) moves the dial step by step around the circle.

• Left
  • Rotates counter-clockwise
• Right
  • Rotates clockwise

It’s a circular path meaning 0 comes after 99 and before 0 comes 99.

The first version of this problem asks,

How many times does the dial end exactly at 0 after a rotation?

Theres really only 4 steps for this:

1. Take our current position (let’s call it cur) on the dial
2. Apply the rotation (either left or right)
3. If after this rotation, we land on 0, we can increment our counter
4. Repeat for all instructions

Why Do We Use Modulo 100?

Since the dial wraps around every 100 numbers, we can use a modulo to ensure our range is within 0 and 99.

Part 2

This section is fairly straightforward and I somehow managed to waste 4 whole minutes on it.

The only difference this time is that when we count every the dial points at 0, we also include during the rotation.

Meaning, if we do R1000 that means we pass 0, 10 times. If we do R60, we pass 0 only 1 time.

So compared to the first version, we have to simulate each movement and check whether we are currently at 0. This is easily achievable through a for loop and we check each time.

Solution

You can find the Github code for this problem here