<p>Given <code>head</code>, the head of a linked list, determine if the linked list has a cycle in it.</p> <p>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 <code>next</code> pointer. Internally, <code>pos</code> is used to denote the index of the node that tail's <code>next</code> pointer is connected to. <strong>Note that <code>pos</code> is not passed as a parameter</strong>.</p> <p>Return <code>true</code><em> if there is a cycle in the linked list</em>. Otherwise, return <code>false</code>.</p> <p> </p> <p><strong>Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="width: 300px; height: 97px; margin-top: 8px; margin-bottom: 8px;" /> <pre> <strong>Input:</strong> head = [3,2,0,-4], pos = 1 <strong>Output:</strong> true <strong>Explanation:</strong> There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed). </pre> <p><strong>Example 2:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="width: 141px; height: 74px;" /> <pre> <strong>Input:</strong> head = [1,2], pos = 0 <strong>Output:</strong> true <strong>Explanation:</strong> There is a cycle in the linked list, where the tail connects to the 0th node. </pre> <p><strong>Example 3:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="width: 45px; height: 45px;" /> <pre> <strong>Input:</strong> head = [1], pos = -1 <strong>Output:</strong> false <strong>Explanation:</strong> There is no cycle in the linked list. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li>The number of the nodes in the list is in the range <code>[0, 10<sup>4</sup>]</code>.</li> <li><code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code></li> <li><code>pos</code> is <code>-1</code> or a <strong>valid index</strong> in the linked-list.</li> </ul> <p> </p> <p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>