165.h

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param l1: ListNode l1 is the head of the linked list
     * @param l2: ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    ListNode * mergeTwoLists(ListNode * l1, ListNode * l2) {
        // write your code here
        ListNode dummyhead(0);
        ListNode *cur = &dummyhead;
        
        while(l1 != nullptr && l2 != nullptr){
            if(l1->val < l2->val){
                cur->next = l1;
                cur = l1;
                l1 = l1->next;
            }
            else{
                cur->next = l2;
                cur = l2;
                l2 = l2->next;
            }
        }
        if(l1) cur->next = l1;
        else   cur->next = l2;
        
        return dummyhead.next;
    }
};