剑指offer:面试题5、从尾到头打印链表

题目描述

输入一个链表，按链表从尾到头的顺序返回一个ArrayList。

代码示例

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class Offer5 {
    public static void main(String[] agrs) {
        //构建链表
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        Offer5 testObj = new Offer5();
        //打印列表
        testObj.printList(head);
        //测试法1
//        List<Integer> res = testObj.printListFromTailToHead(head);
        //测试法2
//        List<Integer> res = testObj.printListFromTailToHead2(head);
        //测试法3
        List<Integer> res = testObj.printListFromTailToHead3(head);
        System.out.println(res);
    }

    //法1：最容易想到的使用栈
    public List<Integer> printListFromTailToHead(ListNode listNode) {
        Stack<Integer> stack = new Stack<>();
        while (listNode != null) {
            stack.add(listNode.val);
            listNode = listNode.next;
        }
        List<Integer> res = new ArrayList<>();
        while (!stack.isEmpty()) {
            res.add(stack.pop());
        }
        return res;
    }

    //法2：头插法逆序链表
    public List<Integer> printListFromTailToHead2(ListNode listNode) {
        ListNode head = new ListNode(-1);
        while (listNode != null) {
            ListNode next = listNode.next;
            listNode.next = head.next;
            head.next = listNode;
            listNode = next;
        }
        List<Integer> res = new ArrayList<>();
        head = head.next;
        while (head != null) {
            res.add(head.val);
            head = head.next;
        }
        return res;
    }

    //法3：使用递归
    public List<Integer> printListFromTailToHead3(ListNode listNode) {
        List<Integer> res = new ArrayList<>();
        if (listNode != null) {
            res.addAll(printListFromTailToHead3(listNode.next));
            res.add(listNode.val);
        }
        return res;
    }

    private void printList(ListNode head) {
        if (head == null) {
            return;
        }
        while (head != null) {
            System.out.println(head.val);
            head = head.next;
        }
    }

    static class ListNode {
        int val;
        ListNode next;
        ListNode(int val) {
            this.val = val;
        }
    }
}