因为迭代比较好理解
所以我们在这里用递归

class Solution {
public:ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {if(!l1) return l2;else if(!l2) 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;}}
};

END