合并两个有序链表

　　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。/**  * Definition for singly-linked list.  * struct ListNode {  *     int val;  *     struct ListNode *next;  * };  */ struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2){     struct ListNode* head = (struct ListNode*)malloc(sizeof(struct ListNode));     head->next = NULL;     struct ListNode* tempNode = head;     while (list1 || list2) {         struct ListNode* node = (struct ListNode*)malloc(sizeof(struct ListNode));         if(list1 && list2) {             if(list1->val <= list2->val) {                 node->val = list1->val;                 node->next = NULL;                 tempNode->next = node;                 tempNode = tempNode->next;                 list1 = list1->next;             }else {                 node->val = list2->val;                 node->next = NULL;                 tempNode->next = node;                 tempNode = tempNode->next;                 list2 = list2->next;             }         }else if (list1 && !list2) {             node->val = list1->val;             node->next = NULL;             tempNode->next = node;             tempNode = tempNode->next;             list1 = list1->next;         }else {             node->val = list2->val;             node->next = NULL;             tempNode->next = node;             tempNode = tempNode->next;             list2 = list2->next;         }     }          return head->next; }