劍指offer 03：從尾到頭打印鏈表

題目描述

輸入一個鏈表，按鏈表值從尾到頭的順序返回一個ArrayList。

遞歸法

/**
*    public class ListNode {
*        int val;
*        ListNode next = null;
*
*        ListNode(int val) {
*            this.val = val;
*        }
*    }
*
*/
import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
         ArrayList<Integer> ret = new ArrayList<>();
        if(listNode != null){
            ret.addAll(printListFromTailToHead(listNode.next));
            ret.add(listNode.val);
        }
        return ret;
    }
}

棧

/**
*    public class ListNode {
*        int val;
*        ListNode next = null;
*
*        ListNode(int val) {
*            this.val = val;
*        }
*    }
*
*/
import java.util.ArrayList;
import java.util.Stack;
public class Solution{
    public ArrayList<Integer> printListFromTailToHead(ListNode listNode){
        Stack<Integer> stack = new Stack<>();
        while(listNode != null){
            stack.add(listNode.val);
            listNode = listNode.next;
        }
        ArrayList<Integer> ret = new ArrayList<>();
        while(!stack.isEmpty())
            ret.add(stack.pop());
        return ret;
    }
}

頭插法

/**
*    public class ListNode {
*        int val;
*        ListNode next = null;
*
*        ListNode(int val) {
*            this.val = val;
*        }
*    }
*
*/
import java.util.ArrayList;
public class Solution{
    public ArrayList<Integer> printListFromTailToHead(ListNode listNode){
       ListNode head = new ListNode(-1);
        while(listNode != null){
            ListNode next = listNode.next;
            listNode.next = head.next;
            head.next = listNode;
            listNode = next;
        }
        
        ArrayList<Integer> ret = new ArrayList<>();
        head = head.next;
        while(head != null){
            ret.add(head.val);
            head = head.next;
        }    
        return ret;
    }
}