Find the Duplicate Number

main.py

class Solution:
    def findDuplicate(self, arr: List[int]) -> int:
        # Awesome proof here: https://youtu.be/9YTjXqqJEFE?t=431
        hare, tortoise = 0, 0
        low_speed = False
        while True:
            if low_speed:
                hare = arr[hare]
            else:
                hare = arr[arr[hare]]
            tortoise = arr[tortoise]
            if hare == tortoise:
                if low_speed:
                    return hare
                hare = 0
                low_speed = True