LeetCode_ 21 (merge two ordered linked lists)

Title Description ： Merge two ascending linked lists into a new Ascending Link list and return . The new linked list is made up of all the nodes of the given two linked lists .

Example 1：

Input ：l1 = [1,2,4], l2 = [1,3,4]
Output ：[1,1,2,3,4,4]

Example 2：
Input ：l1 = [], l2 = []
Output ：[]

Example 3：
Input ：l1 = [], l2 = [0]
Output ：[0]

Tips ：
The range of the number of nodes in the two linked lists is [0, 50]
-100 <= Node.val <= 100
l1 and l2 All according to Non decreasing order array

// recursive 
/**
 * Definition for singly-linked list.
 * public class ListNode {
    
 *     int val;
 *     ListNode next;
 *     ListNode() {
    }
 *     ListNode(int val) {
     this.val = val; }
 *     ListNode(int val, ListNode next) {
     this.val = val; this.next = next; }
 * }
 */
class Solution {
    
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
    
        if(list1==null)return list2;
        if(list2==null)return list1;
        // Put the smaller of the two into the new linked list , The other one is called the big one 
        ListNode list3=list1.val<list2.val?list1:list2;
        // Compare the small back one with the just big one 
        list3.next=mergeTwoLists(list3.next,list1.val>=list2.val?list1:list2);
        return list3;
    }
}