1.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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

2.題目要求：合并兩個排序好的鏈表。

3.方法：遍歷兩個鏈表，每次取最小值。

4.代碼：
class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
ListNode * head = new ListNode(0);
ListNode * ptr = head;

    while(l1 != NULL || l2 != NULL){
        ListNode * cur = NULL;
        if(l1 == NULL){
            cur = l2;
            l2 = l2->next;
        }
        else if(l2 == NULL){
            cur = l1;
            l1 = l1->next;
        }
        else{
            if(l1->val < l2->val){
                cur = l1;
                l1 = l1->next;
            }
            else{
                cur = l2;
                l2 = l2->next;
            }
        }
        ptr->next = cur;
        ptr = ptr->next;
    }
    
    return head->next;
}

};