活动打卡

class Solution {
public:
  ListNode* merge(ListNode* l1, ListNode* l2) {
    ListNode dummy{0};
    auto tail = &dummy;
    while (l1 && l2) {
      if (l1->val < l2->val) {
        tail = tail->next = l1;
        l1 = l1->next;
      } else {
        tail = tail->next = l2;
        l2 = l2->next;
      }
    }
    tail->next = l1 ? l1 : l2;
    return dummy.next;
  }  
};