题目：

题解：

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     struct ListNode *next;* };*/typedef struct ListNode ListNode;
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) {//如果List1和list2中有一个为空就直接返回另一个链表if(list1 == NULL){return list2;}if(list2 == NULL){return list1;}//定义l1,l2指针分别指向list1和list2的头节点ListNode* l1, *l2;ListNode* newhead, *newtail;//给新链表的开辟一个哨兵位newhead = newtail = (ListNode*)malloc(sizeof(ListNode));l1 = list1,l2 = list2;while(l1 && l2){if(l1->val <= l2->val){ newtail->next = l1;newtail = newtail->next;l1 = l1->next;}else{newtail->next = l2;newtail = newtail->next;l2 = l2->next;}}if(l1){newtail->next = l1;}if(l2){newtail->next = l2;}//新链表的第一个节点是头节点为无效数据，因此返回头节点的nextreturn newhead->next;
}