From 1f28fb2d4cd0edc8e2d4d472c897a818b600104d Mon Sep 17 00:00:00 2001
From: linlihong <747682928@qq.com>
Date: Wed, 21 Aug 2024 11:46:04 +0800
Subject: [PATCH] =?UTF-8?q?=E5=9B=9E=E6=96=87=E9=93=BE=E8=A1=A8=E7=A9=BA?=
 =?UTF-8?q?=E9=97=B4=E5=A4=8D=E6=9D=82=E5=BA=A6O(1)=E8=A7=A3=E6=B3=95?=
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---
 .../com/cool/hot100/linkedlist/Num234.java    | 66 +++++++++++++++----
 1 file changed, 53 insertions(+), 13 deletions(-)

diff --git a/src/main/java/com/cool/hot100/linkedlist/Num234.java b/src/main/java/com/cool/hot100/linkedlist/Num234.java
index 9718f0e..c8f2839 100644
--- a/src/main/java/com/cool/hot100/linkedlist/Num234.java
+++ b/src/main/java/com/cool/hot100/linkedlist/Num234.java
@@ -1,5 +1,7 @@
 package com.cool.hot100.linkedlist;
 
+import org.junit.Test;
+
 public class Num234 {
 
     /**
@@ -12,22 +14,60 @@ public class Num234 {
     public ListNode node;
 
     public boolean isPalindrome(ListNode head) {
-        node = head;
-        return reNodeCheck(head);
-    }
+//        node = head;
+//        return reNodeCheck(head);
+        //时间复杂度为O(n),空间复杂度为O(1)解法
+        ListNode slowNode = head;
+        ListNode fastNode = head;
+        while (fastNode.next != null && fastNode.next.next != null) {
+            fastNode = fastNode.next.next;
+            slowNode = slowNode.next;
+        }
+        ListNode temp = head;
+        ListNode reverse = reverse(slowNode.next);
+        while (reverse != null) {
+            if (reverse.val != temp.val) {
+                return false;
+            }
+            temp = temp.next;
+            reverse = reverse.next;
 
-    //递归反向对比
-    public boolean reNodeCheck(ListNode node){
-        if (node!=null){
-            if(!reNodeCheck(node.next)){
-                return false;
-            }
-            if(node.val!=this.node.val){
-                return false;
-            }
-            this.node=this.node.next;
         }
         return true;
     }
 
+    public ListNode reverse(ListNode node) {
+        ListNode pre = null;
+        while (node != null) {
+            ListNode next = node.next;
+            node.next = pre;
+            pre = node;
+            node = next;
+        }
+        return pre;
+    }
+
+    //递归反向对比
+    public boolean reNodeCheck(ListNode node) {
+        if (node != null) {
+            if (!reNodeCheck(node.next)) {
+                return false;
+            }
+            if (node.val != this.node.val) {
+                return false;
+            }
+            this.node = this.node.next;
+        }
+        return true;
+    }
+    @Test
+    public void test(){
+//        ListNode node=new ListNode(5,null);
+        ListNode node1=new ListNode(1,null);
+        ListNode node2=new ListNode(2,node1);
+        ListNode node3=new ListNode(2,node2);
+        ListNode node4=new ListNode(1,node3);
+        isPalindrome(node4);
+    }
+
 }