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();
    }
}