Question.java

package Question2_7;

import CtCILibrary.LinkedListNode;

public class Question {
	public class Result {
		public LinkedListNode node;
		public boolean result;
		public Result(LinkedListNode n, boolean res) {
			node = n;
			result = res;
		}
	}

	public Question.Result isPalindromeRecurse(LinkedListNode head, int length) {
		if (head == null || length == 0) {
			return new Question.Result(null, true);
		} else if (length == 1) {
			return new Question.Result(head.next, true);
		} else if (length == 2) {
			return new Question.Result(head.next.next, head.data == head.next.data);
		}
		Question.Result res = isPalindromeRecurse(head.next, length - 2);
		if (!res.result || res.node == null) {
			return res; // Only "result" member is actually used in the call stack.
		} else {
			res.result = head.data == res.node.data;
			res.node = res.node.next;
			return res;
		}
	}
	
	public boolean isPalindrome(LinkedListNode head) {
		int size = 0;
		LinkedListNode n = head;
		while (n != null) {
			size++;
			n = n.next;
		}
		Result p = isPalindromeRecurse(head, size);
		return p.result;
	}
	
	public static void main(String[] args) {
		int length = 10;
		LinkedListNode[] nodes = new LinkedListNode[length];
		for (int i = 0; i < length; i++) {
			nodes[i] = new LinkedListNode(i >= length / 2 ? length - i - 1 : i, null, null);
		}
		
		for (int i = 0; i < length; i++) {
			if (i < length - 1) {
				nodes[i].setNext(nodes[i + 1]);
			}
			if (i > 0) {
				nodes[i].setPrevious(nodes[i - 1]);
			}
		}
		// nodes[length - 2].data = 9; // Uncomment to ruin palindrome
		
		LinkedListNode head = nodes[0];
		System.out.println(head.printForward());
		Question q = new Question();
		System.out.println(q.isPalindrome(head));
	}

}