leetcode:21. 合并两个有序链表

主要思路是依次比较两个链表的第一个节点的大小，小的就连接到新链表后面，直到其中一个链表为空后，将剩下的节点一起全部连接新链表最后

/**
 * 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 *current = head;
    while(list1 != NULL && list2 != NULL){
        if(list1->val <= list2->val){
            current->next = list1;
            list1 = list1->next;
        }
        else{
            current->next = list2;
            list2 = list2->next;
        }
        current = current->next;
    }
    if(list1 != NULL){
        current->next = list1;
    }
    if(list2 != NULL){
        current->next = list2;
    }
    return head->next;
}