002.链表反转（递归2021-10-22）

 

 

package com.pta.one;

/**
 * 1.链表反转
 */
public class ReverseListCopy
{
    static class ListNode
    {
        int val;
        ListNode next;

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

    //迭代
    public static ListNode iterate(ListNode head)
    {
        /**
         * 1.不用for循环是因为不知到循环长度（列表长度）
         * 2.不用foreach是因为传入的是ListNode,foreach底层依赖迭代器
         */
        ListNode prev = null, next;
        ListNode curr = head;
        while (curr != null)
        {
            next = curr.next;//先保存指针
            curr.next = prev;//prev赋值给下一个next
            prev = curr;
            curr = next;
        }
        return prev;
    }

    //递归
    public static ListNode recursion(ListNode head)
    {
        if (head==null || head.next==null)
        {
            return head;
        }
        //首先找到最后一个元素
        ListNode new_head=recursion(head.next);
        head.next.next = head;
        head.next = null;
        return new_head;
    }


    public static void main(String[] args)
    {
        ListNode node5 = new ListNode(5, null);
        ListNode node4 = new ListNode(4, node5);
        ListNode node3 = new ListNode(3, node4);
        ListNode node2 = new ListNode(2, node3);
        ListNode node1 = new ListNode(1, node2);
        ListNode prev = recursion(node1);
        System.out.println(prev);
    }
}