021 Merge Two Sorted Lists 合并两个有序链表

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
Example:
Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4
详见：https://leetcode.com/problems/merge-two-sorted-lists/description/

Java实现：

非递归：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        if(l1==null&&l2==null){
            return null;
        }
        ListNode head=new ListNode(-1);
        ListNode cur=head;
        while(l1!=null&&l2!=null){
            if(l1.val<l2.val){
                cur.next=l1;
                l1=l1.next;
            }else{
                cur.next=l2;
                l2=l2.next;
            }
            cur=cur.next;
        }
        cur.next=l1==null?l2:l1;
        return head.next;
    }
}

递归：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null&&l2==null){
            return null;
        }
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        ListNode head=null;
        if(l1.val<l2.val){
            head=l1;
            head.next=mergeTwoLists(l1.next,l2);
        }else{
            head=l2;
            head.next=mergeTwoLists(l1,l2.next);
        }
        return head;
    }
}