Merge two sorted linked lists and return it as a newsortedlist. The new list should be made by splicing together the nodes of the first two lists.

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

方法一、遞迴

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        
 
else if(l2 == null) return l1;
        else if(l1.val <= l2.val ){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }

方法二、迭代

當l1和l2都不為空時，取較小的。最後新增剩下l1或l2

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         ListNode dumpy 
 
= new ListNode(0);
        ListNode cur = dumpy;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                cur.next = l1;
                l1 = l1.next;
                cur = cur.next;
            } else {
                cur.next = l2;
                l2 
 
= l2.next;
                cur = cur.next;
            }
        }
        if(l1 != null) cur.next = l1;
        if(l2 != null) cur.next = l2;
        return dumpy.next;
    }

參考連結：

https://leetcode.com/problems/merge-two-sorted-lists/

https://leetcode-cn.com/problems/merge-two-sorted-lists