linked-list-cycle-ii.md

<p>Given a linked list, return the node where the cycle begins. If there is no cycle, return <code>null</code>.</p>

<p>To represent a cycle in the given linked list, we use an integer <code>pos</code> which represents the position (0-indexed)&nbsp;in the linked list where tail connects to. If <code>pos</code> is <code>-1</code>, then there is no cycle in the linked list.</p>

<p><b>Note:</b> Do not modify the linked list.</p>

<p>&nbsp;</p>

<p><strong>Example 1:</strong></p>

<pre>
<strong>Input: </strong>head = [3,2,0,-4], pos = 1
<strong>Output: </strong>tail connects to node index 1
<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the second node.
</pre>

<p><img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="height: 97px; width: 300px;" /></p>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input: </strong>head = [1,2], pos = 0
<strong>Output: </strong>tail connects to node index 0
<strong>Explanation:</strong> There is a cycle in the linked list, where tail connects to the first node.
</pre>

<p><img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="height: 74px; width: 141px;" /></p>

<p><strong>Example 3:</strong></p>

<pre>
<strong>Input: </strong>head = [1], pos = -1
<strong>Output: </strong>no cycle
<strong>Explanation:</strong> There is no cycle in the linked list.
</pre>

<p><img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="height: 45px; width: 45px;" /></p>

<p>&nbsp;</p>

<p><b>Follow-up</b>:<br />
Can you solve it without using extra space?</p>