运行了下
  怎么就循环了一次。有些疑惑？ 求解释  
#include<stdio.h>
#include<malloc.h>
#define NULL 0

struct person
{
    int num;
    struct person *next;
}a;

struct person *input(struct person *head)           //建立链表，对人编号，由1号到13号；
{
    int n=0;
    struct person *p1, *p2;
    p1=p2=(struct person *)malloc(sizeof(struct person));
    p1=&a;
    head=p1;
    printf("please input the bianhao:\n");
    while(++n<=13)
    {
        scanf("%d", &p1->num);
        if(n==13)
        {
            p1->next=NULL;
            break;
        }
        p2=p1;
        p1=(struct person *)malloc(sizeof(struct person));
        p2->next=p1;
    }
    return(head);
}

struct person *search(struct person *head, int n)   //0表示被提出，n记录剩下的人数，当剩一人时就退出while

{
    int k=1;
    struct person *p;
    p=head;
    while(n>1)
    {
            if(p->num!=0)
            {
                k++;
                p=p->next;
            }
            else
            {
                p=p->next;
            }
            if(k==3)
            {
                p->num=0;
                n--;   
                k=1;
                p=p->next;
            }
            if(p->next==NULL)
            {
                p=head;
                while(p->num==0)
                {
                    p=p->next;
                }
            }
    }

    return(head);
}

void main()
{
    struct person *head=NULL, *p=NULL;
    int i;
    head=input(head);
    p=search(head, 13);

    printf("the last number is:\n");
   
    for(i=0;i<13;i++)
    {
        if(p->num!=0)
        {
            printf("%d\n", p->num);
        }
        p=p->next;
    }
}
 

嗯有道理