142.linked-list-cycle-ii.py

#
# @lc app=leetcode id=142 lang=python3
#
# [142] Linked List Cycle II
#

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

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow = fast = head
        while fast and fast.next:
            slow, fast = slow.next, fast.next.next
            if slow == fast:
                while head != slow:
                    head, slow = head.next, slow.next
                return slow
        return None

# @lc code=end