104 lines
2.7 KiB
C++
104 lines
2.7 KiB
C++
#include<stdio.h>
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#include<string.h>
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#include<algorithm>
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#include<vector>
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#include<queue>
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#define ll __int64
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using namespace std ;
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vector<int> vec[44] ;
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const int mod = 1000000007 ;
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int dp[1<<21] ;
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int fa[44] , sz[44] , vis[44] ;
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int to[44] , id[44] , du[44] ;
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int c[44][44] ;
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void init () {
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c[0][0] = 1 ;
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for ( int i = 1 ; i < 44 ; i ++ ) {
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c[i][0] = 1 ;
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for ( int j = 1 ; j <= i ; j ++ )
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c[i][j] = ( c[i-1][j] + c[i-1][j-1] ) % mod ;
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}
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}
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int find ( int a ) {
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return fa[a] == a ? a : fa[a] = find ( fa[a] ) ;
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}
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struct Point {
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int id , st ;
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Point () {}
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Point (int a , int b):id(a),st(b) {}
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} ;
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int gao ( int s , int n , int& re) {
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int tot = 0 ;
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queue<Point> Q ;
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for ( int i = 1 ; i <= n ; i ++ ) {
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if ( find (i) == s && du[i] == 0 ) {
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to[tot++] = 0 ;
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Q.push ( Point(i,1<<tot-1) ) ;
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}
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}
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while ( !Q.empty () ) {
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Point u = Q.front () ;
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int st = u.st ;
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int f = u.id ;
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Q.pop () ;
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for ( int i = 0 ; i < vec[f].size () ; i ++ ) {
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int v = vec[f][i] ;
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du[v] -- ;
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id[v] |= st ;
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if ( du[v] == 0 ) {
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to[tot++] = id[v] ;
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Q.push ( Point(v,id[v]|(1<<(tot-1))) ) ;
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}
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}
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}
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ll ret = c[re][tot] ;
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re -= tot ;
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dp[0] = 1 ;
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for ( int i = 1 ; i < 1<<tot ; i ++ ) {
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dp[i] = 0 ;
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for ( int j = 0 ; j < tot ; j ++ ) {
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if ( ((i&(1<<j))) ) {
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int st = i ^ (1<<j) ;
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if ( (~st)&to[j] ) continue ;
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dp[i] += dp[i^(1<<j)] ;
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dp[i] %= mod ;
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}
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}
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}
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ret *= dp[(1<<tot)-1] ;
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ret %= mod ;
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return ret ;
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}
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int main () {
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init () ;
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int n , m ;
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while ( scanf ( "%d%d" , &n , &m ) != EOF ) {
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int re = n ;
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memset ( vis , 0 , sizeof ( vis ) ) ;
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for ( int i = 1 ; i <= n ; i ++ ) {
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vec[i].clear () ;
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fa[i] = i ;
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du[i] = 0 ;
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id[i] = 0 ;
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}
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for ( int i = 1 ; i <= m ; i ++ ) {
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int a , b ;
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scanf ( "%d%d" , &a , &b ) ;
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vec[a].push_back ( b ) ;
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du[b] ++ ;
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int x = find ( a ) , y = find ( b ) ;
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if ( x != y ) fa[x] = y ;
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}
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ll ans = 1 ;
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for ( int i = 1 ; i <= n ; i ++ ) {
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if ( !vis[find(i)] ) {
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ans *= gao (find(i),n,re) ;
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ans %= mod ;
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vis[find(i)] = 1 ;
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}
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}
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printf ( "%I64d\n" , ans ) ;
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}
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return 0 ;
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}
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