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.

遞歸實現：

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

Leetcode 21. Merge Two Sorted Lists（easy）