Leetcode 141. Linked List Cycle

Description: Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter. Return true if there is a cycle in the linked list. Otherwise, return false.

Link:https://leetcode.com/problems/linked-list-cycle/

Examples:

Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

思路: 没有环的链表终止条件是node.next = None, 有环的链表终止条件是node.next 在已经遍历过的nodes中出现过。

# 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
        """
        if not head: return False
        if not head.next: return False
        
        p = head
        nodes = []
        while p and (p not in nodes):
            nodes.append(p)
            p = p.next
        if not p:
            return False
        return True

日期: 2020-11-21 最近的实验被自己写出很多bug，从烦躁到越来越平静地debug，甚至很快乐