leetcode [64] merge tow sorted lists

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
{
    if (l1 == NULL) return l2;
    if (l2 == NULL) return l1;
    ListNode *tmp = new ListNode(0);
    ListNode *head = tmp;
    while(l1 && l2)
    {
        if (l1 -> val <= l2 -> val)
        {
            tmp -> next = l1;
            tmp = l1;
            l1 = l1 -> next;
        }
        else
        {
            tmp -> next = l2;
            tmp = l2;
            l2 = l2 -> next;
        }
    }
    if (l1)
    {
        tmp -> next = l1;
    }
    else
    {
        tmp -> next = l2;
    }
    return head -> next;
}
};