环形链表

空间复杂度O(1): 快指针追上慢指针即可确定有回环

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

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        
        l1 = head
        l2 = head
        
        while(l2 != None and l2.next != None):
            l1 = l1.next
            l2 = l2.next.next
            if l1 == l2:
                return True
        
        return False

空间复杂度O(N)：比较简单，直接用哈希记录即可