反转单链表和双链表。

解答1[Java]：

核心思想

设计两个指针，一个 prev指向前一个节点 ，一个cur 指向当前节点。最初的时候，prev指向null，cur指向第一个节点。最后 cur 指向 null 的时候，prev 刚好指向最后一个节点。

代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

}

解答2[Java]：

核心思想

主要是利用 head.next.next = head; 相当于把当前节点的下一个节点指向了当前节点。递归调用到最后，p 一直是指向最后一个节点，并且不断被返回到上一层调用的。

代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode p = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return p;
    }
}

拓展：反转双向链表

核心思路

代码

双向链表节点定义

public class DoubleNode {
    public int value;
    public DoubleNode last;
    public DoubleNode next;

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

算法

class Solution {
    public static DoubleNode reverseList(DoubleNode head) {
        DoubleNode pre = null;
        DoubleNode post = null;
        while (head != null) {
            post = head.next;
            head.next = pre;
            head.last = post;
            pre = head;
            head = post;
        }
        return pre;
    }
}

完整测试代码

public class Code_07_ReverseList {
    public static class Node {
        public int value;
        public Node next;

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

    public static Node reverseList(Node head) {
        Node pre = null;
        Node post = null;
        while (head != null) {
            post = head.next;
            head.next = pre;
            pre = head;
            head = post;
        }
        return pre;
    }

    public static class DoubleNode {
        public int value;
        public DoubleNode last;
        public DoubleNode next;

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

    public static DoubleNode reverseList(DoubleNode head) {
        DoubleNode pre = null;
        DoubleNode post = null;
        while (head != null) {
            post = head.next;
            head.next = pre;
            head.last = post;
            pre = head;
            head = post;
        }
        return pre;
    }

    public static void printLinkedList(Node head) {
        System.out.print("Linked List: ");
        while (head != null) {
            System.out.print(head.value + " ");
            head = head.next;
        }
        System.out.println();
    }

    public static void printDoubleLinkedList(DoubleNode head) {
        System.out.print("Double Linked List: ");
        DoubleNode end = null;
        while (head != null) {
            System.out.print(head.value + " ");
            end = head;
            head = head.next;
        }
        System.out.print("| ");
        while (end != null) {
            System.out.print(end.value + " ");
            end = end.last;
        }
        System.out.println();
    }

    public static void main(String[] args) {
        Node head1 = new Node(1);
        head1.next = new Node(2);
        head1.next.next = new Node(3);
        printLinkedList(head1);
        head1 = reverseList(head1);
        printLinkedList(head1);

        DoubleNode head2 = new DoubleNode(1);
        head2.next = new DoubleNode(2);
        head2.next.last = head2;
        head2.next.next = new DoubleNode(3);
        head2.next.next.last = head2.next;
        head2.next.next.next = new DoubleNode(4);
        head2.next.next.next.last = head2.next.next;
        printDoubleLinkedList(head2);
        printDoubleLinkedList(reverseList(head2));

    }

}