Linked List Cycle

Last updated on Jul 2, 2020 Leetcode, linked list

LeetCode 141

Question

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

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.

Follow up:

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

Solution

def hasCycle(head) :
        # to check whether head is NULL or not
        if head is None or head.next is None:
            return False
        # initialised two node with head
        slow, fast = head, head
        while fast and fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next

            if slow == fast:
                return True
        else:
            return False

Summary

Definition of linked list
Two pointers, one slow, the other fast.

LeetCode 142

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

Note: Do not modify the linked list.

Example 1:

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

Example 3:

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

Follow-up: Can you solve it without using extra space?

Leetcode linked list two pointer