Linked List Cycle


#1

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?


#2

Uses two pointers with one moving twice the speed as the other.

func hasCycle(head *ListNode) bool {
    if head==nil {return false}
    slow, fast := head, head
    for fast.Next!=nil && fast.Next.Next!=nil{
        slow = slow.Next
        fast = fast.Next.Next
        if slow==fast{
            return true
        }
    }
   
    return false
}