leetcode 30: 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.

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function

if(!l2) return l1;
if(!l1) return l2;

ListNode* head = NULL;
ListNode** pre = &head;

while( l1 && l2){

if(l1->val < l2->val){
*pre = l1;
l1 = l1->next;
} else {
*pre = l2;
l2 = l2->next;
}

pre = &((*pre)->next);
}

if(l1) *pre = l1;
if(l2) *pre = l2;

return head;

}
};