Method mergeTwoLists

memo/listnode.cpp:21–41

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* preHead = new ListNode(-1);
        ListNode* cur = nullptr;
        ListNode* pre = preHead;
        pre->next = cur;

        while(l1 && l2) {
            if (l1->val <= l2->val) {
                pre->next = l1;
                pre = pre->next;
                l1 = l1->next;
            } else {
                pre->next = l2;
                pre = pre->next;
                l2 = l2->next;
            }
        }
        
        pre->next = l1 == nullptr ? l2 : l1;
        return preHead->next;
    }
};