/*
Given head, the head of a linked list, determine if the linked list has a cycle in it.

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.

Return true if there is a cycle in the linked list. Otherwise, return false.

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

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

Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: 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?
*/

//Definition for singly-linked list.
public class ListNode {
    public var val: Int
    public var next: ListNode?
    public init(_ val: Int) {
        self.val = val
        self.next = nil
    }
}

func hasCycle(_ head: ListNode?) -> Bool {

    // we will use the fast slow Pointer
    var slow = head
    var fast = head
    
    while (fast != nil) && ((fast?.next) != nil) {
        slow = slow?.next
        fast = fast?.next?.next
        if slow === fast {
            return true
        }
    }
    
    return false
}

/*
 Runtime: 68 ms Your runtime beats 14.29 % of swift submissions.
 Memory Usage: 15.4 MB Your memory usage beats 66.03 % of swift submissions.
 */
