将两个链表合并成有序链表

 链接:https://leetcode-cn.com/problems/merge-two-sorted-lists/

//1.哨兵位+双指针法(归并法)


struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2)
{
    if(list1==NULL)
    {
        return list2;
    }
    else if(list2==NULL)
    {
        return list1;
    }
    struct ListNode* head=(struct ListNode*)malloc(sizeof(struct ListNode));
    if(head!=NULL)
    head->next=NULL;
    
    struct ListNode* tail=head;
    struct ListNode* start1=list1;
    struct ListNode* start2=list2;
    while(start1&&start2)
    {
        if((start1->val)<(start2->val))
        {
            tail->next=start1;
            tail=start1;
            start1=start1->next;
        }
        else
        {
            tail->next=start2;
            tail=start2;
            start2=start2->next;
        }
    }
    if(start1==NULL)
    {
        tail->next=start2;
    }
    else if(start2==NULL)
    {
        tail->next=start1;
    }
    struct ListNode* newhead=head->next;
    free(head);
    return newhead;

}