DAY0008.cpp

// 21. Merge Two Sorted Lists
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
    public:
        ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
            if(list2==NULL) return list1;
            if(list1==NULL) return list2;
            ListNode answerlist;
            ListNode *answer=&answerlist;
            while(list1!=NULL&&list2!=NULL){
                if(list1->val<list2->val){
                    answer->next=list1;
                    list1=list1->next;
                }
                else{
                    answer->next=list2;
                    list2=list2->next;
                }
                answer=answer->next;
            }
            answer->next=(list1==NULL?list2:list1);
            return answerlist.next;
        }
    };