# 141. Linked List Cycle
#### Difficulty: Easy
link: https://leetcode.com/problems/linked-list-cycle/

### 1. Hash table
#### $O(n)$ runtime, $O(n)$ space

用set紀錄看過的node，如果有重複就代表有cycle。

##### python

# 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:
        visit = set()
        while head is not None:
            if head in visit:
                return True
            visit.add(head)
            head = head.next
        
        return False

### 2. Two Pointers
#### $O(n)$ runtime, $O(1)$ space

Floyd Cycle Detection Algorithm (龜兔賽跑演算法)，用一快一慢的兩個指標，如果快的指標會追到慢的指標，就代表有cycle。

##### python

# 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:
        slow = fast = head
        while fast is not None and fast.next is not None:
            slow = slow.next
            fast = fast.next.next
            if slow is fast:
                return True
        
        return False

###### tags: `leetcode`