请修改append函数，利用这个函数实现

两个非降序链表的并集，1->2->3 和 2->3->5 并为 1->2->3->5
另外只能输出结果，不能修改两个链表的数据。

#include <stdio.h>#include <stdlib.h>#include <malloc.h>struct Node{    int num;    Node * next;};Node * createTail(){    int x;    Node *head = NULL, *p = NULL, *tail = NULL;    puts("\nplease enter some digits(end of '.'):");    while( scanf("%d",&x) )    {        p = (Node *)malloc(sizeof(Node));        p->num = x;        p->next = NULL;        if( NULL == head )        {            tail = p;            head = tail;        }        else        {            tail->next = p;            tail = p;        }    }    getchar();    return head;}Node * CombinationNode(Node* head1, Node* head2){    Node *head,*tail,*p = head1,*q = head2,*s;        if( NULL == p )        return q;    if( NULL == q )        return p;    tail = p;    if( p->num > q->num)                tail = q;    head = tail;    while( NULL != p && NULL != q )    {        if(p->num <= q->num )        {            s = p;            p = p->next;        }        else        {            s = q;            q = q->next;        }        tail->next = s;        tail = s;    }    if( NULL == p ) p = q;        s = p;    tail->next = s;        return head;}void printHead(Node *head){    if( NULL == head )        return;    printf("List: ");    while(head)    {        printf("%d->",head->num);        head = head->next;    }    puts("NUL");}void main( void ){    Node* head1,*head2,*head;    head1 = createTail();    printHead(head1);    head2 = createTail();    printHead(head2);    head = CombinationNode(head1,head2);    printHead(head);}//////////////////////////////////////////////////////////////////////////////please enter some digits(end of '.'):13579.List: 1->3->5->7->9->NULplease enter some digits(end of '.'):2456789.List: 2->4->5->6->7->8->9->NULList: 1->2->3->4->5->5->6->7->7->8->9->9->NULPress any key to continue//////////////////////////////////////////////////////////////////////////////