dp[i][j]:i代表前i个单位，j代表匹配的方案；

#include
     
      #include
      
       #include
       
        #include
        
         #define ll long long#define MST(vis,x) memset(vis,x,sizeof(vis))#define maxn 21#define mod 998244353using namespace std;int dp[maxn][(1<<20)+1];vector
         
          vec[22];int main(){    int n,m;    while(scanf("%d%d",&n,&m)!=EOF)    {        for(int a=1;a<=n;a++)vec[a].clear();        for(int a=1;a<=m;a++)        {            int x,y;            scanf("%d%d",&x,&y);            vec[x].push_back(y);        }        dp[0][0]=1;        for(int i=1;i<=n;i++)//代表前i个        {            for(int j=0;j