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| 1 | +<h2><a href="https://leetcode.com/problems/linked-list-cycle">Linked List Cycle</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given <code>head</code>, the head of a linked list, determine if the linked list has a cycle in it.</p> |
| 2 | + |
| 3 | +<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> |
| 4 | + |
| 5 | +<p>Return <code>true</code><em> if there is a cycle in the linked list</em>. Otherwise, return <code>false</code>.</p> |
| 6 | + |
| 7 | +<p> </p> |
| 8 | +<p><strong class="example">Example 1:</strong></p> |
| 9 | +<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="width: 300px; height: 97px; margin-top: 8px; margin-bottom: 8px;" /> |
| 10 | +<pre> |
| 11 | +<strong>Input:</strong> head = [3,2,0,-4], pos = 1 |
| 12 | +<strong>Output:</strong> true |
| 13 | +<strong>Explanation:</strong> There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed). |
| 14 | +</pre> |
| 15 | + |
| 16 | +<p><strong class="example">Example 2:</strong></p> |
| 17 | +<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="width: 141px; height: 74px;" /> |
| 18 | +<pre> |
| 19 | +<strong>Input:</strong> head = [1,2], pos = 0 |
| 20 | +<strong>Output:</strong> true |
| 21 | +<strong>Explanation:</strong> There is a cycle in the linked list, where the tail connects to the 0th node. |
| 22 | +</pre> |
| 23 | + |
| 24 | +<p><strong class="example">Example 3:</strong></p> |
| 25 | +<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="width: 45px; height: 45px;" /> |
| 26 | +<pre> |
| 27 | +<strong>Input:</strong> head = [1], pos = -1 |
| 28 | +<strong>Output:</strong> false |
| 29 | +<strong>Explanation:</strong> There is no cycle in the linked list. |
| 30 | +</pre> |
| 31 | + |
| 32 | +<p> </p> |
| 33 | +<p><strong>Constraints:</strong></p> |
| 34 | + |
| 35 | +<ul> |
| 36 | + <li>The number of the nodes in the list is in the range <code>[0, 10<sup>4</sup>]</code>.</li> |
| 37 | + <li><code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code></li> |
| 38 | + <li><code>pos</code> is <code>-1</code> or a <strong>valid index</strong> in the linked-list.</li> |
| 39 | +</ul> |
| 40 | + |
| 41 | +<p> </p> |
| 42 | +<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p> |
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