21.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 *result = nullptr;
        ListNode *current = nullptr;
        while (list1 != nullptr && list2 != nullptr) {
            if (list1 -> val < list2 -> val) {
                if (result == nullptr) {
                    result = current = list1;
                    list1 = list1 -> next;
                    current -> next = nullptr;
                }
                else {
                    //result != nullptr and current != nullptr
                    current -> next = list1;
                    current = current -> next;
                    list1 = list1 -> next;
                    current -> next = nullptr;
                }
            }
            else {
                //list1 -> val >= list2 -> val
                if (result == nullptr) {
                    result = current = list2;
                    list2 = list2 -> next;
                    current -> next = nullptr;
                }
                else {
                    //result != nullptr and current != nullptr
                    current -> next = list2;
                    current = current -> next;
                    list2 = list2 -> next;
                    current -> next = nullptr;
                }
            }
        }
        if (list1 != nullptr) {
            if (result == nullptr) {
                result = list1;
            }
            else {
                //result == current != nullptr
                current -> next = list1;
            }
        }
        else if (list2 != nullptr) {
            if (result == nullptr) {
                result = list2;
            }
            else {
                //result == current != nullptr
                current -> next = list2;
            }
        }
        return result;
    }
};