剑指offer第五题

输入一个链表，从尾到头打印链表每个节点的值。

 但是 根据往常的经验 如果if里面有return了 就不要写else了 

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

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

public class FromTailToHead {
    public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(null==listNode ){
            return list;
        }

        Stack<Integer> stack = new Stack<Integer>();
        ListNode nextNode = listNode;
        while(null!=nextNode){
            stack.push(nextNode.val);
            nextNode = nextNode.next;
        }
        while(!stack.isEmpty()){
            list.add(stack.pop());
        }
        return list;
    }
}

下面是递归调用

/*
 * 递归实现
 */
public class FromTailToHead2 {
    public static void printListFromTailToHead(ListNode Head) {
        if(null!=Head){
            if(null!=Head.next){
                printListFromTailToHead(Head.next);
            }
            System.out.print(Head.val+"\t");
        }
    }

    public static void main(String[] args) {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        n1.next=n2;
        n2.next=n3;
        n3.next=null;
        ListNode Head =n1;
        while(Head!=null){
            System.out.print(Head.val+"\t");
            Head=Head.next;
        }
        System.out.println();
        printListFromTailToHead(n1);

    }
}