<p>Given the <code>head</code> of a linked list, return <em>the node where the cycle begins. If there is no cycle, return </em><code>null</code>.</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 <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 (<strong>0-indexed</strong>). It is <code>-1</code> if there is no cycle. <strong>Note that</strong> <code>pos</code> <strong>is not passed as a parameter</strong>.</p>

<p><strong>Do not modify</strong> the linked list.</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="height: 145px; width: 450px;" />
<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><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png" style="height: 105px; width: 201px;" />
<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><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png" style="height: 65px; width: 65px;" />
<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>&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>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode* slow=head;
        ListNode* fast=head;
        while(fast!=NULL and fast->next!=NULL){
            slow=slow->next;
            fast=fast->next->next;
            if(slow==fast){
                slow=head;
                while(slow!=fast){
                    slow=slow->next;
                    fast=fast->next;
                }
                return slow; //starting point
            }
        }
        return NULL;
    }
};
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