141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1 Output: true Explanation: There is a cycle in the linked list, where tail connects to the second node. assets.leetcode.com/uploads/201…

Example 2:

Input: head = [1,2], pos = 0 Output: true Explanation: There is a cycle in the linked list, where tail connects to the first node. assets.leetcode.com/uploads/201…

Example 3:

Input: head = [1], pos = -1 Output: false Explanation: There is no cycle in the linked list. assets.leetcode.com/uploads/201…

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

思路：快慢指针，慢指针追上快指针，就说明有环

python:

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        try:
            fast = head
            slow = head
            while(1):
                if fast.next.next and slow.next:
                    fast = fast.next.next
                    slow = slow.next
                    if fast is slow or fast.next is slow:
                        return True
                else:
                    return False
        except:
            return False