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"content": "<p>Given the <code>head</code> of a linked list, return <em>the node where the cycle begins. If there is no cycle, return </em><code>null</code>.</p>\n\n<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>0-indexed</strong>). It is <code>-1</code> if there is no cycle. <strong>Note that</strong> <code>pos</code> <strong>is not passed as a parameter</strong>.</p>\n\n<p><strong>Do not modify</strong> the linked list.</p>\n\n<p> </p>\n<p><strong>Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png\" style=\"height: 145px; width: 450px;\" />\n<pre>\n<strong>Input:</strong> head = [3,2,0,-4], pos = 1\n<strong>Output:</strong> tail connects to node index 1\n<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the second node.\n</pre>\n\n<p><strong>Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png\" style=\"height: 105px; width: 201px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2], pos = 0\n<strong>Output:</strong> tail connects to node index 0\n<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the first node.\n</pre>\n\n<p><strong>Example 3:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png\" style=\"height: 65px; width: 65px;\" />\n<pre>\n<strong>Input:</strong> head = [1], pos = -1\n<strong>Output:</strong> no cycle\n<strong>Explanation:</strong> There is no cycle in the linked list.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of the nodes in the list is in the range <code>[0, 10<sup>4</sup>]</code>.</li>\n\t<li><code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code></li>\n\t<li><code>pos</code> is <code>-1</code> or a <strong>valid index</strong> in the linked-list.</li>\n</ul>\n\n<p> </p>\n<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>\n",
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