CTCI 2.7

Implement a function to check if a linked list is a palindrome.

Reverse the second half of the list and then compare it with the first half.

/* Assume that we can destroy the list, reverse the second half of the list and compare it with the first
half. If we were not permit to destroy the list, change the list to an array and then check if it is a
palidrome.
*/

public class IsPalidrome {
    public boolean isPalidrome(Node head) {
        if(head == null || head.next == null) return true;
        int cnt = 0;
        Node temp = head;
        while(temp != null) { cnt++; temp = temp.next; }
        int half = cnt / 2;
        temp = head;
        while(half > 1) { temp = temp.next; half--; }
        Node sent = new Node(0);
        //break the list into two parts
        Node temp1 = temp.next, temp2 = temp.next; temp.next = null;
        //reverse the second half
        while(temp1 != null) {
            temp2 = temp1.next;
            temp1.next = sent.next;
            sent.next = temp1;
            temp1 = temp2;
        }
        //check if it is palidrome
        Node head1 = head, head2 = sent.next;
        while(head1 != null && head2 != null) {
            if(head1.val == head2.val) {
                head1 = head1.next;
                head2 = head2.next;
            }
            else return false;
        }
        return true;
    }

    public static void main(String[] args) {
        Node node1 = new Node(1); Node node11 = new Node(1); node1.next = node11;
        Node node2 = new Node(1); Node node22 = new Node(2); node2.next = node22;
        Node node31 = new Node(1); Node node32 = new Node(2); Node node33 = new Node(1); node31.next = node32; node32.next = node33;
        Node node41 = new Node(1); Node node42 = new Node(3); Node node43 = new Node(2); Node node44 = new Node(1);
        node41.next = node42; node42.next = node43; node43.next = node44;
        IsPalidrome ip = new IsPalidrome();
        System.out.println(ip.isPalidrome(node1));
        System.out.println(ip.isPalidrome(node2));
        System.out.println(ip.isPalidrome(node31));
        System.out.println(ip.isPalidrome(node41));
    }
}

CTCI 2.7