0142.py

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def findDuplicate(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums) > 1:
            slow = nums[0]
            fast = nums[nums[0]]
            while slow != fast:
                slow = nums[slow]
                fast = nums[nums[fast]]
                
            entry = 0
            while entry != slow:
                entry = nums[entry]
                slow = nums[slow]
                
            return entry
        
        return -1