Recursion

Recursion for linked list

234. Palindrome Linked List

Java Solution 1

    public boolean isPalindrome(ListNode head) {
        return head == null || recurse (head, head) != null;
    }

    private ListNode recurse (ListNode node, ListNode head) {
        if (node == null) return  head;
        ListNode res = recurse (node.next, head);
        if (res == null) return res;
        else if (res.val == node.val) return (res.next == null ? res : res.next);
        else return null;
    }

Java Solution 2

public class Solution {
    public ListNode root;
    public boolean isPalindrome(ListNode head) {
        root = head;
        return (head == null) ? true : _isPalindrome(head);
    }

    public boolean _isPalindrome(ListNode head) {
        boolean flag = true;
        if (head.next != null) {
            flag = _isPalindrome(head.next);
        }
        if (flag && root.val == head.val) {
            root = root.next;
            return true;
        }
        return false;
    }
}