合併兩個排序好的列表，遞迴演算法pass

/**
 * 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 == nullptr) return l2;
        else if(l2 == nullptr) return l1;
        ListNode * p;
        if(l1->val < l2->val){
             p = l1;
             p -> next = mergeTwoLists(l1->next,l2);
        }
        else{
             p = l2;
             p -> next = mergeTwoLists(l1,l2->next);
        }
        return p;
        
        
    }
};