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Description
tab: English
<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'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> </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> </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> <= Node.val <= 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> </p>
<p><strong>Follow up:</strong> Can you solve it using <code>O(1)</code> (i.e. constant) memory?</p>
---
[submissions](https://leetcode.com/problems/linked-list-cycle-ii/submissions/) | [solutions](https://leetcode.com/problems/linked-list-cycle-ii/solutions/)
tab: 中文
<p>给定一个链表的头节点 <code>head</code> ，返回链表开始入环的第一个节点。 <em>如果链表无环，则返回 <code>null</code>。</em></p>
<p>如果链表中有某个节点，可以通过连续跟踪 <code>next</code> 指针再次到达，则链表中存在环。 为了表示给定链表中的环，评测系统内部使用整数 <code>pos</code> 来表示链表尾连接到链表中的位置（<strong>索引从 0 开始</strong>）。如果 <code>pos</code> 是 <code>-1</code>，则在该链表中没有环。<strong>注意：<code>pos</code> 不作为参数进行传递</strong>，仅仅是为了标识链表的实际情况。</p>
<p><strong>不允许修改 </strong>链表。</p>
<ul>
</ul>
<p> </p>
<p><strong>示例 1：</strong></p>
<p><img src="https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png" /></p>
<pre>
<strong>输入：</strong>head = [3,2,0,-4], pos = 1
<strong>输出：</strong>返回索引为 1 的链表节点
<strong>解释：</strong>链表中有一个环，其尾部连接到第二个节点。
</pre>
<p><strong>示例 2：</strong></p>
<p><img alt="" src="https://assets.leetcode-cn.com/aliyun-lc-upload/uploads/2018/12/07/circularlinkedlist_test2.png" /></p>
<pre>
<strong>输入：</strong>head = [1,2], pos = 0
<strong>输出：</strong>返回索引为 0 的链表节点
<strong>解释：</strong>链表中有一个环，其尾部连接到第一个节点。
</pre>
<p><strong>示例 3：</strong></p>
<p><img alt="" src="https://assets.leetcode-cn.com/aliyun-lc-upload/uploads/2018/12/07/circularlinkedlist_test3.png" /></p>
<pre>
<strong>输入：</strong>head = [1], pos = -1
<strong>输出：</strong>返回 null
<strong>解释：</strong>链表中没有环。
</pre>
<p> </p>
<p><strong>提示：</strong></p>
<ul>
<li>链表中节点的数目范围在范围 <code>[0, 10<sup>4</sup>]</code> 内</li>
<li><code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code></li>
<li><code>pos</code> 的值为 <code>-1</code> 或者链表中的一个有效索引</li>
</ul>
<p> </p>
<p><strong>进阶：</strong>你是否可以使用 <code>O(1)</code> 空间解决此题？</p>
---
[提交记录](https://leetcode.cn/problems/linked-list-cycle-ii/submissions/) | [题解](https://leetcode.cn/problems/linked-list-cycle-ii/solution/)
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