package com.hit.basmath.learn.linked_list;

import com.hit.common.ListNode;

/**
 * 206. Reverse Linked List
 * <p>
 * Reverse a singly linked list.
 * <p>
 * Example:
 * <p>
 * Input: 1->2->3->4->5->NULL
 * <p>
 * Output: 5->4->3->2->1->NULL
 * <p>
 * Follow up:
 * <p>
 * A linked list can be reversed either iteratively or recursively. Could you implement both?
 */
public class _206 {
    /**
     * iterative solution
     */
    public ListNode reverseList(ListNode head) {
        ListNode newHead = null;
        while (head != null) {
            ListNode nextNode = head.next;
            head.next = newHead;
            newHead = head;
            head = nextNode;
        }
        return newHead;
    }

    /**
     * recursive solution
     */
    public ListNode reverseList2(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode nextNode = head.next;
        ListNode newHead = reverseList2(nextNode);
        nextNode.next = head;
        head.next = null;
        return newHead;
    }
}