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+<h2><a href="https://leetcode.com/problems/linked-list-cycle">141. Linked List Cycle</a></h2><h3>Easy</h3><hr><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&nbsp;<code>next</code>&nbsp;pointer. Internally, <code>pos</code>&nbsp;is used to denote the index of the node that&nbsp;tail&#39;s&nbsp;<code>next</code>&nbsp;pointer is connected to.&nbsp;<strong>Note that&nbsp;<code>pos</code>&nbsp;is not passed as a parameter</strong>.</p>
+
+<p>Return&nbsp;<code>true</code><em> if there is a cycle in the linked list</em>. Otherwise, return <code>false</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">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 class="example">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 class="example">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>&nbsp;</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> &lt;= Node.val &lt;= 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>&nbsp;</p>
+<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>