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☕今日份刷题：合并两个有序链表

 题目描述：

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例1：

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

示例2：

输入：l1 = [], l2 = []
输出：[]

示例3：

输入：l1 = [], l2 = [0]
输出：[0]

题目分析： 

这题可太经典了，直接取小的尾插 

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     struct ListNode *next;* };*/
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2){if(list1 == NULL){return list2;}if(list2 == NULL){return list1;}struct ListNode* head = NULL,*tail = NULL;while(list1 && list2){//取小的尾插if(list1->val < list2->val){if(tail == NULL){head = tail = list1;}else{tail->next = list1;tail = tail->next;}list1 = list1->next;}else{if(tail == NULL){head = tail = list2;}else{tail->next = list2;tail = tail->next;}list2 = list2->next;}}if(list1){tail->next = list1;}if(list2){tail->next = list2;}return head;
}