链表反转

原地反转

/**
*    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> list = new ArrayList<>();
        if(listNode==null){
            return list;
        }
        ListNode prev = listNode;
        ListNode curr = listNode.next;
        ListNode next = curr.next;
        prev.next = null;
        
        while(curr!=null){
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        
        while(prev!=null){
            list.add(prev.val);
            prev = prev.next;
        }
        return list;
    }
}

利用 List add（0，） 方法

/**
*    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> list = new ArrayList<>();
        while(listNode!=null){
            list.add(0,listNode.val);
            listNode = listNode.next;
        }
        return list;
    }
}