思路：这题就是双指针，然后如果指针A和指针B所指向的数据是一样的，就一起往后跳，如果不相同，比如A指向的数据大，就把A指针向后移动，如果B指向的数据大，就把B向后移，while 的条件是A和B都没到达链表的结尾。

代码如下：

static class ListNode {
        int data;
        ListNode next;
        ListNode(int val) {
            this.data = val;
        }
    }
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        while (cin.hasNext()) {
            int n = cin.nextInt();
            ListNode list1 = new ListNode(0);
            ListNode cur = list1;
            for (int i = 0; i < n; i++) {
                int a = cin.nextInt();
                cur.next = new ListNode(a);
                cur = cur.next;
            }
            int m = cin.nextInt();
            ListNode list2 = new ListNode(0);
            cur = list2;
            for (int i = 0; i < m; i++) {
                int b = cin.nextInt();
                cur.next = new ListNode(b);
                cur = cur.next;
            }
            ListNode res = find(list1.next,list2.next);//把公共的部分打印出来
            while (res != null){
                int ans = res.data;
                System.out.print(ans+" ");
                res = res.next;
            }
            System.out.println();
        }
    }

    public static ListNode find(ListNode point1, ListNode point2) {
        if (point1 == null || point2 == null) return null;
        ListNode head = new ListNode(-1);
        ListNode pre = head;
        while (point1 != null && point2 != null) {
            if (point1.data == point2.data) {
                ListNode newNode = new ListNode(point1.data);
                point1 = point1.next;
                point2 = point2.next;
                pre.next = newNode;
                pre = pre.next;
            }else if(point1.data > point2.data){
                point1 = point1.next;
            }else {
                point2 = point2.next;
            }
        }
        return head.next;
    }

改进，，其看了别人的方法，发现其实这题根本用不到链表，数组也是一样可以完成的，如下：

package tengxun;

import java.util.Scanner;

/**
 * Created by IntelliJ IDEA.
 *
 * @Author: 
 * @Email: 
 * @Date: 2020/9/6
 * @Time: 20:03
 * @Version: 1.0
 * @Description: Description
 */
public class First {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr1 = new int[n];
        for (int i = 0; i < n; i++) {
            arr1[i] = sc.nextInt();
        }
        int m = sc.nextInt();
        int[] arr2 = new int[m];
        for (int i = 0; i < m; i++) {
            arr2[i] = sc.nextInt();
        }
        sc.close();
        int p1 = 0, p2 = 0;
        while (p1 < n && p2 < m) {
            if (arr1[p1] == arr2[p2]) {
                System.out.print(arr1[p1] + " ");
                p1++;
                p2++;
            } else if (arr1[p1] < arr2[p2]) {
                p2++;
            } else {
                p1++;
            }
        }
    }
}