linked-list-cycle-ii

Linked List Cycle II

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detectCycle

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

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&#39;s <code>next</code> pointer is connected to (0-indexed). It is <code>-1</code> if there is no cycle. Note that <code>pos</code> is not passed as a parameter.

Do not modify the linked list.

&nbsp;
Example 1:
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" style="height: 145px; width: 450px;" />
<pre>
Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
</pre>

Example 2:
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="height: 105px; width: 201px;" />
<pre>
Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
</pre>

Example 3:
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="height: 65px; width: 65px;" />
<pre>
Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
</pre>

&nbsp;
Constraints:

<ul>
	<li>The number of the nodes in the list is in the range <code>[0, 104]</code>.</li>
	<li><code>-105 &lt;= Node.val &lt;= 105</code></li>
	<li><code>pos</code> is <code>-1</code> or a valid index in the linked-list.</li>
</ul>

&nbsp;
Follow up: Can you solve it using <code>O(1)</code> (i.e. constant) memory?

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