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PalindromeLinkedList.java
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PalindromeLinkedList.java
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package Leetcode;
/**
* @author kalpak
*
* 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?
*
*/
public class PalindromeLinkedList {
public static boolean isPalindrome(ListNode head) {
ListNode fast = head;
ListNode slow = head;
while (fast != null && fast.next != null) {
fast = fast.next.next;
slow = slow.next;
}
if (fast != null) { // odd nodes.
slow = slow.next;
}
slow = reverseList(slow);
fast = head;
while (slow != null) {
if (fast.val != slow.val)
return false;
fast = fast.next;
slow = slow.next;
}
return true;
}
private static ListNode reverseList(ListNode head) {
if(head == null)
return head;
ListNode current = head;
ListNode prev = null;
ListNode next = null;
while(current != null) {
// Store the next pointer and then change the next pointer to previous
next = current.next;
current.next = prev;
// prev now moves to the current node
prev = current;
// current goes to the next node which has been cached
current = next;
}
// In the end, now current is null, but prev has the last node.
// Move head to prev.
head = prev;
return head;
}
public static void main(String[] args) {
ListNode l1 = new ListNode(1);
l1.next = new ListNode(2);
l1.next.next = new ListNode(3);
l1.next.next.next = new ListNode(2);
l1.next.next.next.next = new ListNode(1);
System.out.println(isPalindrome(l1));
}
}