287. Find the Duplicate Number

class Solution:
    def findDuplicate(self, A: List[int]) -> int:
        slow = fast = A[0]

        while True:
            slow = A[slow]
            fast = A[A[fast]]
            if slow == fast:
                break
        
        slow = A[0]
        while slow != fast:
            slow = A[slow]
            fast = A[fast]
        
        return slow