LeetCode 141 – Python 3 (Week 2 – 10)

Solution 1

# 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):#the input is head.We do not have pos.
        """
        :type head: ListNode
        :rtype: bool
        """
        s = set()
        
        while head and head.next:
            if head in s:
                return True
            else:
                s.add(head)
                head = head.next
        return False 

Time complexity is O(n). Space complexity is O(n).

Solution 2

# 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):#the input is head.We do not have pos.
        """
        :type head: ListNode
        :rtype: bool
        """
        slow, fast = head, head
        
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
            
        return False

Time complexity is O(n). Space complexity is O(1).