LeetCode 21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

I personally prefer to add a dummy node. But need to remember to delete it after. This will cause memory overflow in some cases.

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(!l1) return l2;
if(!l2) return l1;
ListNode* dummy = new ListNode(0);
ListNode* newHead = dummy;
ListNode* l1_node = l1;
ListNode* l2_node = l2;
while(l1_node && l2_node) {
if(l1_node->val <= l2_node->val) {
ListNode* l1_next = l1_node->next;
dummy->next = l1_node;
dummy = l1_node;
l1_node = l1_next;
} else {
ListNode* l2_next = l2_node->next;
dummy->next = l2_node;
dummy = l2_node;
l2_node = l2_next;
}
}
if(l1_node) dummy->next = l1_node;
if(l2_node) dummy->next = l2_node;
ListNode* head = newHead->next;
delete newHead;
return head;
}