141. Linked List Cycle

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

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        fast = head
        slow = head
        while(fast):
            if fast.next is None:
                return False
            fast = fast.next.next # When using linked list, always checking whether each next is valid or not.
            slow = slow.next
            if fast == slow:
                return True
        return False

Looping criterion is only related to the fast pointer, since it always meets the end before slow.

There are three conditions to consider:

1. There is only one node in linked list

2. There are more than one node and the fast node reaches the final loop:

   • fast jumps two steps each time, it could be the tail node

   • Or it could be the tail.next (None)

Note: When using linked list, always checking whether each next is valid or not.