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PalindromeLinkedList.java
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PalindromeLinkedList.java
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// Copyright (c) 2015 Elements of Programming Interviews. All rights reserved.
package com.epi;
public class PalindromeLinkedList {
// @include
public static boolean isLinkedListAPalindrome(ListNode<Integer> L) {
if (L == null) {
return true;
}
// Finds the second half of L.
ListNode<Integer> slow = L, fast = L;
while (fast != null && fast.next != null) {
fast = fast.next.next;
slow = slow.next;
}
// Compare the first half and the reversed second half lists.
ListNode<Integer> firstHalfIter = L;
ListNode<Integer> secondHalfIter
= ReverseLinkedListIterative.reverseLinkedList(slow);
while (secondHalfIter != null && firstHalfIter != null) {
if (secondHalfIter.data != firstHalfIter.data) {
return false;
}
secondHalfIter = secondHalfIter.next;
firstHalfIter = firstHalfIter.next;
}
return true;
}
// @exclude
public static void main(String[] args) {
if (args.length >= 1) {
ListNode<Integer> head = null;
// Input the node's value in reverse order.
for (String element : args) {
ListNode<Integer> curr
= new ListNode<>(Integer.parseInt(element), head);
head = curr;
}
System.out.println(((isLinkedListAPalindrome(head)) ? "Yes" : "No"));
}
assert(isLinkedListAPalindrome(null));
assert(isLinkedListAPalindrome(new ListNode<>(1, null)));
assert(isLinkedListAPalindrome(new ListNode<>(1, new ListNode<>(1, null))));
assert(
!isLinkedListAPalindrome(new ListNode<>(1, new ListNode<>(2, null))));
assert(!isLinkedListAPalindrome(new ListNode<>(
1, new ListNode<>(3, new ListNode<>(2, new ListNode<>(1, null))))));
ListNode<Integer> head = null;
// A link list is a palindrome.
for (int i = 6; i >= 1; --i) {
ListNode<Integer> curr = new ListNode<>(1, head);
head = curr;
}
assert(isLinkedListAPalindrome(head));
// Still a palindrome linked list.
head = null;
for (int i = 5; i >= 1; --i) {
ListNode<Integer> curr = new ListNode<>(1, head);
head = curr;
}
head.next.next.data = 3;
assert(isLinkedListAPalindrome(head));
// Not a palindrome linked list.
head = null;
for (int i = 5; i >= 1; --i) {
ListNode<Integer> curr = new ListNode<>(i, head);
head = curr;
}
assert(!isLinkedListAPalindrome(head));
}
}