From 301a8b39116187fe10351e9f302d94eef32691ba Mon Sep 17 00:00:00 2001
From: ehlxr <ehlxr.me@gmail.com>
Date: Sun, 4 Dec 2022 22:25:27 +0800
Subject: [PATCH] Palindrome Linked List

---
 .../algorithm/linkedlist/Palindrome.java      | 90 ++++++++++++-------
 1 file changed, 60 insertions(+), 30 deletions(-)

diff --git a/budd-common/src/main/java/io/github/ehlxr/algorithm/linkedlist/Palindrome.java b/budd-common/src/main/java/io/github/ehlxr/algorithm/linkedlist/Palindrome.java
index 7c58f0d..6fd4307 100644
--- a/budd-common/src/main/java/io/github/ehlxr/algorithm/linkedlist/Palindrome.java
+++ b/budd-common/src/main/java/io/github/ehlxr/algorithm/linkedlist/Palindrome.java
@@ -25,11 +25,70 @@
 package io.github.ehlxr.algorithm.linkedlist;
 
 /**
+ * 判断单链表是否是回文
+ *
  * @author ehlxr
  * @since 2022-12-03 23:11.
  */
 public class Palindrome {
-    public static class ListNode {
+    public static void main(String[] args) {
+        ListNode head = new ListNode(1);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(3);
+        head.next.next.next = new ListNode(2);
+        head.next.next.next.next = new ListNode(1);
+        System.out.println(head);
+        System.out.println(isPalindrome(head));
+
+        head = new ListNode(1);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(3);
+        head.next.next.next = new ListNode(3);
+        head.next.next.next.next = new ListNode(2);
+        head.next.next.next.next.next = new ListNode(1);
+        System.out.println(head);
+        System.out.println(isPalindrome(head));
+
+    }
+
+    /**
+     * slow, fast 双指针，slow 前进一步，fast 前进两步，当 fast 到达链表尾部时，slow 处于链表中间
+     * 与此同时反转链表前半部分（head 到中间节点部分）
+     */
+    public static boolean isPalindrome(ListNode head) {
+        if (head == null) {
+            return false;
+        }
+
+        ListNode slow = head, fast = head, pre = null;
+        while (fast != null && fast.next != null) {
+            fast = fast.next.next;
+
+            ListNode tmp = slow.next;
+            slow.next = pre;
+            pre = slow;
+            slow = tmp;
+            // slow = slow.next;
+        }
+        if (fast != null) {
+            // 节点个数为奇数
+            slow = slow.next;
+        }
+        System.out.println("中间节点：" + slow);
+        System.out.println("中位点反转：" + pre);
+
+        while (pre != null) {
+            if (pre.val != slow.val) {
+                return false;
+            }
+            pre = pre.next;
+            slow = slow.next;
+        }
+
+        return true;
+    }
+
+    private static class ListNode {
         int val;
         ListNode next;
 
@@ -53,33 +112,4 @@ public class Palindrome {
                     '}';
         }
     }
-
-    public static boolean isPalindrome(ListNode head) {
-        if (head == null) {
-            return false;
-        }
-
-        ListNode slow = head;
-        ListNode fast = head;
-        while (fast != null && fast.next != null) {
-            slow = slow.next;
-            fast = fast.next.next;
-        }
-        System.out.println("中位节点：" + slow);
-
-        return false;
-    }
-
-    public static void main(String[] args) {
-        ListNode head = new ListNode(1);
-        head.next = new ListNode(2);
-        head.next.next = new ListNode(3);
-        head.next.next.next = new ListNode(4);
-        head.next.next.next.next = new ListNode(5);
-        // head.next.next.next.next.next = new ListNode(6);
-
-        System.out.println(head);
-
-        isPalindrome(head);
-    }
 }