四刷142

Author: diguage

docs/0142-linked-list-cycle-ii.adoc

@@ -1,49 +1,51 @@
 [#0142-linked-list-cycle-ii]
-= 142. Linked List Cycle II
+= 142. 环形链表 II
 
-:stem: latexmath
+https://leetcode.cn/problems/linked-list-cycle-ii/[LeetCode - 142. 环形链表 II ^]
 
-{leetcode}/problems/linked-list-cycle-ii/[LeetCode - Linked List Cycle II^]
+给定一个链表的头节点 `head` ，返回链表开始入环的第一个节点。 _如果链表无环，则返回 `null`。_
 
-Given a linked list, return the node where the cycle begins. If there is no cycle, return `null`.
+如果链表中有某个节点，可以通过连续跟踪 `next` 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 `pos` 来表示链表尾连接到链表中的位置（*索引从 0 开始*）。如果 `pos` 是 `-1`，则在该链表中没有环。*注意：`pos` 不作为参数进行传递*，仅仅是为了标识链表的实际情况。
 
-To represent a cycle in the given linked list, we use an integer `pos` which represents the position (0-indexed) in the linked list where tail connects to. If `pos` is `-1`, then there is no cycle in the linked list.
+*不允许修改* 链表。
 
-*Note:* Do not modify the linked list.
-
-.Example 1:
-[source]
-----
-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.
-----
+*示例 1：*
 
 image::images/0142-01.png[{image_attr}]
 
-.Example 2:
-[source]
-----
-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.
-----
+....
+输入：head = [3,2,0,-4], pos = 1
+输出：返回索引为 1 的链表节点
+解释：链表中有一个环，其尾部连接到第二个节点。
+....
+
+*示例 2：*
 
 image::images/0142-02.png[{image_attr}]
 
-.Example 3:
-[source]
-----
-Input: head = [1], pos = -1
-Output: no cycle
-Explanation: There is no cycle in the linked list.
-----
+....
+输入：head = [1,2], pos = 0
+输出：返回索引为 0 的链表节点
+解释：链表中有一个环，其尾部连接到第一个节点。
+....
+
+*示例 3：*
 
 image::images/0142-03.png[{image_attr}]
 
-*Follow-up:*
+....
+输入：head = [1], pos = -1
+输出：返回 null
+解释：链表中没有环。
+....
 
-Can you solve it without using extra space?
+*提示：*
+
+* 链表中节点的数目范围在范围 `[0, 10^4^]` 内
+* `-10^5^ \<= Node.val \<= 10^5^`
+* `pos` 的值为 `-1` 或者链表中的一个有效索引
+
+**进阶：**你是否可以使用 stem:[O(1)] 空间解决此题？
 
 == 思路分析
 
@@ -116,6 +118,15 @@ include::{sourcedir}/_0142_LinkedListCycleII_2.java[tag=answer]
 include::{sourcedir}/_0142_LinkedListCycleII_3.java[tag=answer]
 ----
 --
+
+四刷::
++
+--
+[{java_src_attr}]
+----
+include::{sourcedir}/_0142_LinkedListCycleII_4.java[tag=answer]
+----
+--
 ====
 
 

logbook/202503.adoc

@@ -250,6 +250,11 @@ endif::[]
 |{doc_base_url}/0143-reorder-list.adoc[题解]
 |✅ 双指针+链表反正+链表合并
 
+|{counter:codes2503}
+|{leetcode_base_url}/linked-list-cycle-ii/[142. 环形链表 II^]
+|{doc_base_url}/0142-linked-list-cycle-ii.adoc[题解]
+|✅ 双指针+数学
+
 |===
 ˚
 截止目前，本轮练习一共完成 {codes2503} 道题。

src/main/java/com/diguage/algo/leetcode/_0142_LinkedListCycleII_4.java

@@ -0,0 +1,34 @@
+package com.diguage.algo.leetcode;
+
+import com.diguage.algo.util.ListNode;
+
+public class _0142_LinkedListCycleII_4 {
+  // tag::answer[]
+  /**
+   * @author D瓜哥 · https://www.diguage.com
+   * @since 2025-04-14 20:27:39
+   */
+  public ListNode detectCycle(ListNode head) {
+    if (head == null || head.next == null) {
+      return null;
+    }
+    ListNode slow = head, fast = head;
+    while (fast != null && fast.next != null) {
+      slow = slow.next;
+      fast = fast.next.next;
+      if (slow == fast) {
+        break;
+      }
+    }
+    if (slow != fast) {
+      return null;
+    }
+    slow = head;
+    while (slow != fast) {
+      slow = slow.next;
+      fast = fast.next;
+    }
+    return slow;
+  }
+  // end::answer[]
+}