21.cpp

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode *head = nullptr, *curr = nullptr, *h;
        if (!l1) return l2;
        if (!l2) return l1;
        if (l1->val < l2->val) {
            head = l1;
            l1 = l1->next;
        } else {
            head = l2;
            l2 = l2->next;
        }
        curr = head;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                curr->next = l1;
                l1 = l1->next;
            } else {
                curr->next = l2;
                l2 = l2->next;
            }
            curr = curr->next;
        }
        if (!l1)
            curr->next = l2;
        else if (!l2)
            curr->next = l1;
        return head;
    }
};