234 Palindrome Linked List

Author: barisaydinoglu

234-palindrome-linked-list/README.md

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+## [234. Palindrome Linked List](https://leetcode.com/problems/palindrome-linked-list/)
+
+| Difficulty | Topics | Acceptance | Total Accepted | Total Submissions |
+| :-: | :-: | --: | --: | --: |
+| Easy | [Linked List](https://leetcode.com/tag/linked-list/), [Two Pointers](https://leetcode.com/tag/two-pointers/) | 35.88% | 252.5K | 703.7K |
+
+### Problem:
+
+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?
+
+| Similar Questions |
+| --- |
+| [Palindrome Number](https://leetcode.com/problems/palindrome-number/) |
+| [Valid Palindrome](https://leetcode.com/problems/valid-palindrome/) |
+| [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/) |

234-palindrome-linked-list/Solution.java

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+/*-*
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public boolean isPalindrome(ListNode head) {
+        ListNode fast = head, slow = head;
+        while (fast != null && fast.next != null) {
+            fast = fast.next.next;
+            slow = slow.next;
+        }
+        if (fast != null) { // odd nodes: let right half smaller
+            slow = slow.next;
+        }
+        slow = reverse(slow);
+        fast = head;
+        while (slow != null) {
+            if (fast.val != slow.val) {
+                return false;
+            }
+            fast = fast.next;
+            slow = slow.next;
+        }
+        return true;
+    }
+
+    public ListNode reverse(ListNode head) {
+        ListNode prev = null;
+        while (head != null) {
+            ListNode next = head.next;
+            head.next = prev;
+            prev = head;
+            head = next;
+        }
+        return prev;
+    }
+}