add 234

Author: andavid

README.md

@@ -49,7 +49,7 @@
 | 19   | [Remove Nth Node From End of List][019]     |
 | 206  | [Reverse Linked List][206]                  |
 | 21   | [Merge Two Sorted Lists][021]               |
-
+| 234  | [Palindrome Linked List][234]               |
 
 
 **其他**
@@ -89,3 +89,4 @@
 [019]: https://github.com/andavid/leetcode-java/blob/master/note/019/README.md
 [206]: https://github.com/andavid/leetcode-java/blob/master/note/206/README.md
 [021]: https://github.com/andavid/leetcode-java/blob/master/note/021/README.md
+[234]: https://github.com/andavid/leetcode-java/blob/master/note/234/README.md

note/234/README.md

@@ -0,0 +1,62 @@
+# [Palindrome Linked List][title]
+
+## Description
+
+Given a singly linked list, determine if it is a palindrome.
+
+**Follow up:**
+Could you do it in O(n) time and O(1) space?
+
+
+## 思路
+
+使用快慢两个指针找到链表中点，慢指针每次前进一步，快指针每次前进两步。在慢指针前进的过程中，同时修改其 next 指针，使得链表前半部分反序。最后比较中点两侧的链表是否相等。
+
+## [完整代码][src]
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+  public boolean isPalindrome(ListNode head) {
+    if (head == null || head.next == null) {
+      return true;
+    }
+
+    ListNode prev = null;
+    ListNode slow = head;
+    ListNode fast = head;
+
+    while (fast != null && fast.next != null) {
+      fast = fast.next.next;
+      ListNode next = slow.next;
+      slow.next = prev;
+      prev = slow;
+      slow = next;
+    }
+
+    if (fast != null) {
+      slow = slow.next;
+    }
+
+    while (slow != null) {
+      if (slow.val != prev.val) {
+        return false;
+      }
+      slow = slow.next;
+      prev = prev.next;
+    }
+
+    return true;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/palindrome-linked-list
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_234/Solution.java

src/com/andavid/leetcode/_234/Solution.java

@@ -0,0 +1,58 @@
+class Solution {
+
+  public static boolean isPalindrome(ListNode head) {
+    if (head == null || head.next == null) {
+      return true;
+    }
+
+    ListNode prev = null;
+    ListNode slow = head;
+    ListNode fast = head;
+
+    while (fast != null && fast.next != null) {
+      fast = fast.next.next;
+      ListNode next = slow.next;
+      slow.next = prev;
+      prev = slow;
+      slow = next;
+    }
+
+    if (fast != null) {
+      slow = slow.next;
+    }
+
+    while (slow != null) {
+      if (slow.val != prev.val) {
+        return false;
+      }
+      slow = slow.next;
+      prev = prev.next;
+    }
+
+    return true;
+  }
+
+  public static class ListNode {
+    int val;
+    ListNode next;
+    ListNode(int x) {
+      val = x;
+    }
+  }
+
+  public static void main(String[] args) {
+    ListNode node1 = new ListNode(1);
+    ListNode node2 = new ListNode(2);
+    ListNode node3 = new ListNode(3);
+    ListNode node4 = new ListNode(2);
+    ListNode node5 = new ListNode(1);
+    node1.next = node2;
+    node2.next = node3;
+    node3.next = node4;
+    node4.next = node5;
+    node5.next = null;
+
+    System.out.println(isPalindrome(node1));
+  }
+
+}