Merge Two Sorted Lists

题目：

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.

思路：

从最前面开始比较，每次都是最一开始的进行比较，但要注意比较过程中检查是否有链表已经为空。

代码：

/**
 * 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) {
        ListNode *dummy=new ListNode(0),*tail=dummy;
        
        while(l1!=NULL&&l2!=NULL){
            if(l1->val<l2->val){
                tail->next=l1;
                l1=l1->next;
            }else{
                tail->next=l2;
                l2=l2->next;
            }
            tail=tail->next;
        }
        //检测还有没有剩余
        if(l1){
            tail->next=l1;
        }
        if(l2){
            tail->next=l2;
        }
        ListNode *head=dummy->next;
        delete dummy;
        return head;
    }
};