[LeetCode]#141. Linked List Cycle

Environment: Python 3.7

Key technique: Fast and slow pointer, .next

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Analysis:

Use fast and slow pointer method to check this problem. If fast pointer (move 2 step) and slow pointer (move 1 step) can meet, we can say it is a cycle.

Solution:

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        slow = head
        fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast: return True
        return False

Submitted result:

Lesson learn:

Python 3.7 . next occur ‘list_iterator’ object has no attribute ‘next’ and we can use .__next__() to solve this issue. However, I still can’t solve the Stopiteration and it need time to fix.

Reference:

https://leetcode.com/problems/linked-list-cycle/discuss/542743/Easy-Python-Solution