LeetCode-141 環形鏈表

• 問題
  給定一個鏈表，判斷鏈表中是否有環。
  進階：
  你能否不使用額外空間解決此題？

• 解答

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

# Solution 1
class Solution(object):
    def hasCycle(self, head):
        nodes = set([])
        cur = head
        while cur and cur.next:
            nodes.add(cur)
            cur = cur.next
            if cur in nodes:
                return True
        return False
# Solution 2 
class Solution(object):
    def hasCycle(self, head):
        fast, slow = head, head
        while fast and fast.next and slow:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                return True
        return False