linked-list-cycle

Linked List Cycle

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hasCycle

Given <code>head</code>, the head of a linked list, determine if the linked list has a cycle in it.

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;Note that&nbsp;<code>pos</code>&nbsp;is not passed as a parameter.

Return&nbsp;<code>true</code> if there is a cycle in the linked list. Otherwise, return <code>false</code>.

&nbsp;
Example 1:
<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>
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
</pre>

Example 2:
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="width: 141px; height: 74px;" />
<pre>
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
</pre>

Example 3:
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="width: 45px; height: 45px;" />
<pre>
Input: head = [1], pos = -1
Output: false
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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