PaliandromeList.java

package linked_list;

/**
 * Created by gouthamvidyapradhan on 25/02/2017.
 * Given a singly linked list, determine if it is a palindrome.
 * <p>
 * Follow up:
 * Could you do it in O(n) time and O(1) space?
 */
public class PaliandromeList {
    public static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
            next = null;
        }
    }

    ListNode headNode;

    /**
     * Main method
     *
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        ListNode node1 = new ListNode(1);
        //ListNode node2 = new ListNode(2);
        //ListNode node3 = new ListNode(3);
        //ListNode node4 = new ListNode(3);
        //ListNode node5 = new ListNode(2);
        //ListNode node6 = new ListNode(1);
        //node1.next = node2;
        //node2.next = node3;
        //node3.next = node5;
        //node4.next = node5;
        //node5.next = node6;
        System.out.println(new PaliandromeList().isPalindrome(node1));
    }

    public boolean isPalindrome(ListNode head) {
        int length = 0, count = 0;
        if (head == null) return true;
        ListNode temp = head;
        while (temp != null) {
            temp = temp.next;
            length++;
        }
        if (length == 1) return true;

        int halfLen = length / 2;

        temp = head;
        while (count < halfLen - 1) {
            temp = temp.next;
            count++;
        }

        ListNode newHead = temp.next;
        temp.next = null;
        reverse(newHead);
        ListNode first = head, second = headNode;
        for (int i = 0; i < halfLen; i++) {
            if (first.val != second.val)
                return false;
            first = first.next;
            second = second.next;
        }
        return true;
    }

    private ListNode reverse(ListNode node) {
        if (node.next == null) {
            headNode = node;
            return node;
        }
        ListNode prev = reverse(node.next);
        node.next = null;
        prev.next = node;
        return node;
    }

}