solve 234.palindrome-linked-list

Author: andavid

vscode/234.palindrome-linked-list.java

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+/*
+ * @lc app=leetcode id=234 lang=java
+ *
+ * [234] Palindrome Linked List
+ *
+ * https://leetcode.com/problems/palindrome-linked-list/description/
+ *
+ * algorithms
+ * Easy (35.33%)
+ * Total Accepted:    247.5K
+ * Total Submissions: 692.5K
+ * Testcase Example:  '[1,2]'
+ *
+ * Given a singly linked list, determine if it is a palindrome.
+ *
+ * Example 1:
+ *
+ *
+ * Input: 1->2
+ * Output: false
+ *
+ * Example 2:
+ *
+ *
+ * Input: 1->2->2->1
+ * Output: true
+ *
+ * Follow up:
+ * Could you do it in O(n) time and O(1) space?
+ *
+ */
+/**
+ * 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;
+  }
+}
+