141.环形链表|快慢指针

• 我的思路：原地修改结点的next，将其指向自己，以此标记是否遍历过

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

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        if not head or not head.next: return False # 特殊情况
        
        ptr = head
        while ptr:
            pre = ptr
            ptr = ptr.next
            pre.next = pre # 将已经遍历过的结点的next指向自己
            if ptr.next == None: return False # 最后一个指向None那就没有环
            if ptr.next == ptr: # 最后一个指向（指向自己的结点）那就有环
                return True
            
        return False

• 快慢指针

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

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        if not head or not head.next: return False
    
        slow = head
        fast = head.next
        while slow != fast:
            if not fast or not fast.next:
                return False
            slow = slow.next
            fast = fast.next.next
        return True
        

• 用额外空间集合标记遍历与否

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

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        sets = set()
        while head:
            if head in sets: return True
            sets.add(head)
            head = head.next
        return False
  

就很奇怪…T^T

参考链接：官方解析