图书馆里有没有某本书，现有6人排队，有3个人来借此书，3个人来还此书，且若到某人借书时，若无书，则立即离开，多少种排队方法能借到书？

#include<stdio.h>#include<stdlib.h>#define SUM 6int state[SUM];//1代表还书 -1代表借书//void book(int book_lend,int book_return,int num,int count){    //算法结束的条件是：借书的人都借到了,即book_lend==0    if (book_lend == 0)    {        for (int i = 0; i < SUM; i++)        {            printf("%d * ",state[i]);        }        printf("\n");        printf("\b\b\b\b   ");    }    //还书的人都还完了，并且还有书的时候，只能借书    if (book_return == 0 && count>0)    {        state[num] = -1;        book(book_lend - 1, book_return, num + 1, count-1);    }    //可借的条件是图书馆有书，可还的条件是有的人没有还      if (count>0&&book_return>0)    {        state[num] = -1;        book(book_lend - 1, book_return, num + 1, count-1);        state[num] = 1;        book(book_lend, book_return - 1, num + 1, count+1);    }    //图书馆的书的总数为0，并且有人没还书，只能还书    if (book_return > 0&&count==0)    {        state[num] = 1;        book(book_lend, book_return - 1, num + 1, 1+count);    }}int main(void){    book(3,3,0,0);    return 0;}