merge two lists

merge.cpp

/**
 * 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) {
        ListNode *sentinel = new ListNode();
        ListNode *tail = sentinel;
        while (list1 && list2) {
            if (list1->val > list2->val) swap(list1, list2);
            tail->next = list1;
            tail = tail->next;
            list1 = list1->next;
        }
        tail->next = list1 ? list1 : list2;
        return sentinel->next;
    }
};