ReverseList.java

package com.haobin.leetcode.linkedlist;

import org.omg.PortableServer.LIFESPAN_POLICY_ID;

/**
 * @Author HaoBin
 * @Create 2020/1/15 15:38
 * @Description: 反转链表
 **/
public class ReverseList {

    public static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
        }
    }


    /**
     * 不借助其他工具，如栈
     * 迭代法
     * 使用两个指针，使当前链表指针指向前一个节点
     * 需要暂时保存当前节点下一个节点的位置
     */
    public static ListNode reverseList1(ListNode head) {
        ListNode pre = null;
        ListNode current = head;
        while (current != null) {
            ListNode next = current.next;
            current.next = pre;
            pre = current;
            current = next;
        }
        return pre;
    }

    /**
     * 递归解法
     * @param head
     * @return
     */
    public static ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode cur = reverseList1(head.next);
        head.next.next  = head;
        head.next = null;
        return cur;
    }

    public static void main(String[] args) {
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        ListNode node4 = new ListNode(4);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        ListNode head = reverseList(node1);
        while (head!=null) {
            System.out.println(head.val);
            head = head.next;
        }
    }
}