/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        if(!l1 || !l2) return l1 ? l1 : l2;     //判断是否其中一个为空，有则返回不是空的那一个
        ListNode *head = new ListNode(0); 
        ListNode *tail = head, *a = l1, *b = l2;     //定义一个哑节点，tail指向待插入位置的前一个位置，a,b分别指向两个链表
        while(a && b)
        {
            if(a -> val < b -> val) 
            {
                tail -> next = a;
                a = a -> next;
            }
            else
            {
                tail -> next = b;
                b = b -> next;
            }
            tail = tail -> next;
        }
        tail -> next = a ? a : b;
        return head -> next;
    }  
};