動機

複習判圈與找loop起點

Problem

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

Notice that you should not modify the linked list.

 

Example 1:

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

Example 2:

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

Example 3:

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

 

Constraints:

  • The number of the nodes in the list is in the range [0, 104].
  • -105 <= Node.val <= 105
  • pos is -1 or a valid index in the linked-list.

 

Follow up: Can you solve it using O(1) (i.e. constant) memory?

Sol

要記的把b的條件寫好，不然會在[1,2],-1的case掛掉誤以為有loop

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        if head is None:
            return None
        a = head
        b = head
        while b and b.next and b.next.next:
            a = a.next
            b = b.next.next
            if a is b:
                break

        if a is b and b and b.next and b.next.next:
            a = head
            while a is not b:
                a = a.next
                b = b.next
            return a
        else:
            return None