21. merge two sorted lists

struct ListNode {
  int val;
  ListNode *next;
  ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
  ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    ListNode head(0);
    ListNode* ptr = &head;
    while (l1 && l2) {
      if (l1->val < l2->val) {
        ptr->next = l1;
        ptr = l1;
        l1 = l1->next;
      } else {
        ptr->next = l2;
        ptr = l2;
        l2 = l2->next;
      }
    }
    if (l1) {
      ptr->next = l1;
    } else if (l2) {
      ptr->next = l2;
    }
    return head.next;
  }
};