【leetcode】206.Reverse Linked List

206.Reverse Linked List

Reverse a singly linked list.

Tips：实现从尾到头打印一个链表。

以下代码包括两张方法：

①使用栈实现，不改变原链表。先读入，后输出。典型的先进后出，可用栈来实现。

②改变原来链表的指针方向。

package easy;

import java.util.Stack;
import easy.ListNode;

public class L206ReverseLinkedList {
    /**
     * Definition for singly-linked list.
     * 
     * public class ListNode { int val; ListNode next; ListNode(int x) { val =
     * x; } }
     */

    public ListNode reverseList(ListNode head) {
        if (head == null)
            return head;
        Stack<Integer> myStack = new Stack<Integer>();
        ListNode pNode = head;
        while (pNode != null) {
            myStack.push(pNode.val);
            pNode = pNode.next;
        }
        int size = myStack.size();
        ListNode newHead = new ListNode(myStack.peek());
        myStack.pop();
        if (size == 1)
            return newHead;
        ListNode node = new ListNode(myStack.peek());
        newHead.next = node;
        myStack.pop();
        while (!myStack.empty()) {
            node.next = new ListNode(myStack.peek());
            myStack.pop();
            node = node.next;
        }
        return newHead;
    }

    public ListNode reverseList2(ListNode head) {
        // 通过改变链表的指针来从尾到头打印链表。
        if (head == null || head.next == null)
            return head;
        ListNode pre = null;
        ListNode cur = head;
        while (cur != null) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }

    public void print(ListNode phead) {
        while (phead != null) {
            System.out.println(phead.val);
            phead = phead.next;
        }
    }

    public static void main(String[] args) {
        ListNode head1 = new ListNode(1);
        ListNode head2 = new ListNode(2);
        ListNode head3 = new ListNode(3);
        ListNode head4 = new ListNode(4);
        head1.next = head2;
        head2.next = head3;
        head3.next = head4;
        head4.next = null;
        L206ReverseLinkedList cc = new L206ReverseLinkedList();
        System.out.println("使用栈");
        ListNode p1 = cc.reverseList(head1);
        cc.print(p1);
        System.out.println("改变指针");
        ListNode p2 = cc.reverseList2(head1);
        cc.print(p2);

    }
}