【UVA11174】Stand in a Line——逆元+树形Dp

All the people in the byteland want to stand in a line in such a way that no person stands closer to the
front of the line than his father. You are given the information about the people of the byteland. You
have to determine the number of ways the bytelandian people can stand in a line.

Input

First line of the input contains T (T < 14) the number of test case. Then following lines contains T
Test cases.
Each test case starts with 2 integers n (1 ≤ n ≤ 40000) and m (0 ≤ m < n). n is the number
of people in the byteland and m is the number of people whose father is alive. These n people are
numbered 1 … n. Next m line contains two integers a and b denoting that b is the father of a. Each
person can have at most one father. And no person will be an ancestor of himself.

Output

For each test case the output contains a single line denoting the number of different ways the soldier
can stand in a single line. The result may be too big. So always output the remainder on dividing ther
the result by 1000000007.

Sample Input

3
3 2
2 1
3 1
3 0
3 1
2 1

Sample Output

2
6
3

题意：村子里有n个人，有多少种方式可以把他们排成一列，使没有人在他的父亲前面，（有的人的父亲不在村子里），输出方案数对1000000007的余数。

分析：村民由父子之间的关系组成一个森林，虽然有多课树，但是我们可以用一个虚拟的根节点将其转化为一棵树，那么根节点的各子树之间相互独立， Dp[i][0]表示以i为根节点的子树的排列的数目，Dp[i][1]表示以i为根节点的树的节点的个数，
那么Dp[Root][0]=Dp[i1][0]×Dp[i2][0]×⋯×Dp[ie][0]×(∑k=1eDp[ik][1])!∏k=1eDp[ik][1]!Dp[Root][1]=∑k=1eDp[ik][1] ,就可以递推出最后的结果，对于除法，可以提前处理出逆元。

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <iostream>

using namespace std;

typedef long long LL;

const LL Mod = 1e9+7;

const int Max = 41000;

typedef struct node
{

    int v,next;
}edge;

edge e[Max*10];

int H[Max],top;

int Du[Max];

LL Dp[Max][2];

LL Inv[Max],Fac[Max];

LL Pow(LL a,LL b) //快速幂求逆元。
{
    LL ans = 1;

    while(b)
    {
        if(b%2)
        {
            ans = (ans*a) % Mod;
        }

        a = (a * a) % Mod;

        b >>= 1;
    }

    return ans;
}

void Init() //处理阶乘和逆元
{
    Inv[0 ]= Fac[0] = 1;

    for(LL i = 1;i<Max;i++)
    {
        Fac[i] = (Fac[i-1]*i)%Mod;

        Inv[i] = Pow(Fac[i],Mod-2);
    }
}

void AddEdge(int u,int v)
{
    e[top].v = v; e[top].next  = H[u];

    H[u] = top++;
}

void DFS(int u)
{
    LL num = 0;

    LL ans = 1;

    for(int i = H[u];~i;i = e[i].next)
    {
        DFS(e[i].v);

        num+=Dp[e[i].v][1];

        ans = (((ans*Dp[e[i].v][0])%Mod)*Inv[Dp[e[i].v][1]])%Mod;
    }

    ans = (ans*Fac[num])%Mod;

    num++;

    Dp[u][0] = ans;

    Dp[u][1] = num;
}

int main()
{
    int n,m,u,v,T;

    Init();

    scanf("%d",&T);

    while(T--)
    {
        scanf("%d %d",&n,&m);

        memset(H,-1,sizeof(H));

        memset(Du,0,sizeof(Du));

        top = 0;

        for(int i = 1;i<=m;i++)
        {
            scanf("%d %d",&u,&v);

            Du[u]++;

            AddEdge(v,u);
        }

        for(int i = 1;i<=n;i++)
        {
            if(!Du[i]) AddEdge(0,i);
        }

        memset(Dp,0,sizeof(Dp));

        DFS(0);

        printf("%lld\n",Dp[0][0]);
    }
    return 0;
}