You are given the head of a singly linked list. Your task is to determine whether the linked list contains a cycle. Return true if a cycle exists; otherwise, return false.
Example 1
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: The last node points to the node at index 1, forming a cycle.
Example 2
Input: head = [1,2], pos = 0
Output: true
Explanation: The last node points back to the first node, forming a cycle.
Example 3
Input: head = [1], pos = -1
Output: false
Explanation: The linked list has no cycle.
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