java递归写法

• 思路参考leetcode
• https://leetcode-cn.com/problems/merge-two-sorted-lists/solution/yi-kan-jiu-hui-yi-xie-jiu-fei-xiang-jie-di-gui-by-/

import java.util.*;
//定义链表
class ListNode{
    int val;
    ListNode next;
    ListNode(){};
    ListNode(int val){
        this.val=val;
    }
    ListNode(int val,ListNode next){
        this.val=val;
        this.next=next;
    }
}

public class Main{
//定义main函数
    public static void main(String[] args){
        Scanner sc =new Scanner(System.in);
        ListNode list1=createList(sc.nextLine().split(" "));
        ListNode list2=createList(sc.nextLine().split(" "));
        ListNode list=combineList(list1,list2);
      //输出
        while(list!=null){
            System.out.print(list.val+" ");
            list=list.next;
        }
    }
  //创建链表
    public static ListNode createList(String [] str){
      //边界条件
        if(str==null||str.length==0){
            return null;
        }
      //建立节点
        ListNode pre=new ListNode(0);
      		//预留头节点，用以return
        ListNode list=pre;
      //循环读取相应文本转换为数字放入listz中
        for(int i=0;i<str.length;i++){
            list.next=new ListNode(Integer.parseInt(str[i]));
            list=list.next;
        }
      //返回头节点
        return pre.next;
    }
  //
    public static ListNode combineList(ListNode l1,ListNode l2){
      //边界条件
       	if(l1==null) return l2;
       	else if(l2==null) return l1;
      //进行递归
      	else if(l1.val<l2.val){
            l1.next=combineList(l1.next,l2);
            return l1;
        }
        else{
            l2.next=combineList(l2.next,l1);
            return l2;
        }
        
    }
}