find-the-duplicate-number.py

class Solution:
    def findDuplicate(self, nums: List[int]) -> int:
        fast = nums[nums[0]]
        slow = nums[0]
        while fast != slow:
            fast = nums[nums[fast]]
            slow = nums[slow]
        slow = 0
        while fast != slow:
            fast = nums[fast]
            slow = nums[slow]
        return slow