21. 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.

分析

先创建一个空结点当做哨位结点，merge_tail指向合并链表的尾部，比较l1和l2，循环合并，最后判断哪个链表先到头，merge_tail->next就指向另一个链表，此处判断用a?b:c比用两个if语句节约时间。

/**
* 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||l2==NULL)
return l1==NULL?l2:l1;
ListNode* merge_head=new ListNode(0);
ListNode* merge_tail=merge_head;
while(l1!=NULL&&l2!=NULL)
{
if(l1->val>l2->val)
{
merge_tail->next=l2;
merge_tail=merge_tail->next;
l2=l2->next;
}
else
{
merge_tail->next=l1;
merge_tail=merge_tail->next;
l1=l1->next;
}
}
merge_tail->next=l1==NULL?l2:l1;
return merge_head->next;
}
};