bzoj 1211: [HNOI2004]树的计数

prufer的应用。。

详细见这篇博客：https://www.cnblogs.com/dirge/p/5503289.html

 import java.math.BigInteger;
 import java.util.*;
 public class Main {
     static long n, h;
     public static void main(String[] args) {
         Scanner in = new Scanner(System.in);
         BigInteger f[] = new BigInteger[151];
         f[0] = BigInteger.ONE;
 
         int sum  = 0;
         for(int i = 1; i <= 150; i++) {
             f[i] = f[i - 1].multiply(BigInteger.valueOf(i));
         }
 
         int n = in.nextInt();
 
         if(n == 1) {
             int x = in.nextInt();
             if(x == 0) System.out.println("1");
             else System.out.println("0");
         } else {
             BigInteger ans = f[n - 2];
             boolean flag = true;
             for(int i = 1; i <= n; i++) {
                 int x = in.nextInt();
                 sum += x;
                 if(x == 0 || x > 150) {
                     flag = false;
                 }
                 else {
                     ans = ans.divide(f[x - 1]);
                 }
             }
             if(flag && sum == (n - 1) * 2) System.out.println(ans);
             else System.out.println("0");
         }
         in.close();
     }
 }