反转单向和双向链表

反转单向和双向链表
【题目】 分别实现反转单向链表和反转双向链表的函数。
【要求】 如果链表长度为N，时间复杂度要求为O(N)，额外空间复杂度要求为O(1)

public class ReverseList {
	// 单向链表节点
	public static class Node {
		public int value;
		public Node next;

		public Node(int value) {
			this.value = value;
		}
	}

	// 双向链表节点
	public static class DoubleNode {
		public int value;
		public DoubleNode last;
		public DoubleNode next;

		public DoubleNode(int value) {
			this.value = value;
		}
	}

	// 反转单向链表
	public static Node reverseList(Node head) {
		Node pre = null;
		Node next = null;
		while (head != null) {
			next = head.next;
			head.next = pre;
			pre = head;
			head = next;
		}
		return pre;
	}

	// 反转双向链表
	public static DoubleNode reverseList(DoubleNode head) {
		DoubleNode next = null;
		DoubleNode pre = null;
		while (head != null) {
			next = head.next;
			head.next = pre;
			head.last = next;
			pre = head;
			head = next;
		}
		return pre;
	}
}