141. 环形链表、142. 环形链表 II

news2025/11/4 23:49:14

题目
在这里插入图片描述
思路
链表无环情况：有空结点
链表有环：有些结点会重复所以用集合（哈希表）来记录遍历的结点结点不存在，则将结点加到集合中，当遍历到的结点存在集合中，即为链表环开始的结点，即有环
代码

# 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 head is None:
            return False
        if head.next is None:
            return False
        s =set()
        p = head
        while p:
            if p.next is None:
                return False
            if p in s:
                return True
            else:
                s.add(p)
                p=p.next

142. 环形链表 II
在这里插入图片描述
思路跟上一题一样，只不过是输出链表环开始的节点

代码

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

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None:
            return None
        if head.next is None:
            return None
        s= set()
        p = head
        while p:
            if p.next is None:
                return None
            if p in s:
                return p
            s.add(p)
            p=p.next