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.

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).


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.


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.

Difficulty: introductory

Starter code:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        

Solutions:
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if head == None:
            return False
        
        slow = head
        fast = head.next
        
        while slow != fast:
            if fast is None or fast.next is None:
                return False
            slow = slow.next
            fast = fast.next.next
        
        return True

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

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

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        nodes_seen = set()
        while head is not None:
            if head in nodes_seen:
                return True
            nodes_seen.add(head)
            head = head.next
        return False

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        cur = head
        check = set()
        while cur:
            if cur in check:
                return True
            check.add(cur)
            cur = cur.next
        return False

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        try:
            slow = head
            fast = head.next 
            while slow is not fast:
                slow = slow.next
                fast = fast.next.next
            return True
        except:
            return False

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if head is None:
            return False

        slow, fast = head, head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
        return False