141. Linked List Cycle

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

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, 10^4].
• -10^5 <= Node.val <= 10^5
• pos is -1 or a valid index in the linked-list.

Approach

Solution

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


class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        fast = slow = head

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

Explanation