#include <stdio.h>
#include <stdlib.h>
typedef struct player
{
    int accont;
    char a[20];
    int power;
    struct player *next;
}PLAYER;
PLAYER *build(int num)
{
    PLAYER *prept,*head;
    int i;
    printf("input information:\n");
    PLAYER *pt=(PLAYER *)malloc(sizeof(PLAYER));
    if(pt!=NULL)
    {
        scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
    }
    else
    {
        printf("error!\n");
        exit(0);
    }
    for(i=1;i<num;i++)
    {
        PLAYER *pt=(PLAYER *)malloc(sizeof(PLAYER));
        if(pt!=NULL)
        {
           scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
        }
        else
        {
            printf("error!\n");
            exit(0);
        }
    }
    return head;
}
PLAYER *fidelast(PLAYER *head)
{
    PLAYER *pt=head;
    while(pt->next!=NULL)
    {
        pt=pt->next;
    }
    return pt;
}
PLAYER *fidenode(PLAYER *head,int num)
{
    int i;
    PLAYER *pt=head;
    for(i=1;i<num&&pt->next!=NULL;i++)
    {
        pt=pt->next;
    }
    return pt;
}
PLAYER *insertlast(PLAYER *head)
{
    PLAYER *prept,*pt;
    pt=(PLAYER *)malloc(sizeof(PLAYER));
    if(pt!=NULL)
        scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
    else
        exit(0);
    prept=fidelast(head);
    prept->next=pt;
    pt->next=NULL;
    return head;
}
PLAYER *insert(PLAYER *head,int num)
{
    PLAYER *prept,*pt;
    pt=(PLAYER *)malloc(sizeof(PLAYER));
    if(pt!=NULL)
        scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
    else
        exit(0);
    prept=fidenode(head,num);
    pt->next=prept->next;
    prept->next=pt;
    return head;
}

int main()
{
    PLAYER *q,*c;
    int a;
    printf("input how many:\n");
    scanf("%d",&a);
    q=build(a);
  
    printf("input what you wan fide:\n");
    scanf("%d",&a);
    c=fidenode(q,a);
    printf("%s",c->next);

}

如果head为NULL ，那么pt为NULL，但是pt->next的值是什么？

你的build子程序head指针并未赋值，完了还是野指针。

#include <stdio.h>
#include <stdlib.h>
typedef struct player
{
    int accont;
    char a[20];
    int power;
    struct player *next;
}PLAYER;
PLAYER *build(int num)
{
    PLAYER *prept,*head,*pt;
    int i;
    printf("input information:\n");
    pt=(PLAYER *)malloc(sizeof(PLAYER));
    if(pt!=NULL)
    {
        scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
        head=pt;
        prept=pt;
    }
    else
    {
        printf("error!\n");
        exit(0);
    }
    for(i=1;i<num;i++)
    {
        PLAYER *pt=(PLAYER *)malloc(sizeof(PLAYER));
        if(pt!=NULL)
        {
           scanf("%d%s%d",&pt->accont,pt->a,&pt->power);
           prept->next=pt;
           prept=pt;
        }
        else
        {
            printf("error!\n");
            exit(0);
        }
    }
    return head;
}

PLAYER *fidenode(PLAYER *head,int num)
{
    int i=1;
    PLAYER *pt=head;
    while(i<num&&pt!=NULL)
    {
        pt=pt->next;
        i++;
    }
    return pt;
}

int main()
{
    PLAYER *q,*c;
    int a;
    printf("input how many:\n");
    scanf("%d",&a);
    q=build(a);
    printf("input what you wan fide:\n");
    scanf("%d",&a);
    c=fidenode(q,a);
    printf("%s",c->next);

}
我把指针的问题改了后为什么还是不对呢，请解释一下，谢谢

printf("%s",c->next->a); //这里改下，你是要输出字符串
第二张图是越界，只有三个节点，第三个节点的next是什么？

谢谢大佬