[4교 합동훈련] 나무 자르기 게임. - 나무 DP. - 조합 수학.

#include
#include
#include

#define R register
#define ll long long
#define max_n 100010
#define mod 1000000007
using namespace std;
struct ED{int to,nex;}edge[200100];
int n,et;
int col[max_n],st[max_n];
ll f[max_n][2];

int read()
{
    R int xx;R char ch;
    while(ch=getchar(),ch<'0'||ch>'9');xx=ch-'0';
    while(ch=getchar(),ch>='0'&&ch<='9')xx=xx*10+ch-'0';
    return xx;
}

void exgcd(ll &x,ll &y,ll a,ll b)
{
    if(b==0)x=1,y=0;
    else
    {
        exgcd(x,y,b,a%b);
        ll tmp=x;
        x=(y+mod)%mod;
        y=(tmp-(a/b)*y%mod+mod)%mod;
    }
}

void dfs(int u,int pre)
{
    R int e,v;
    R ll x,y;
    R ll sum1=0,sum2=1;
    for(e=st[u];e!=-1;e=edge[e].nex)
    if((v=edge[e].to)!=pre)
    {
        dfs(v,u);
        sum2=sum2*(f[v][1]+f[v][0])%mod;
    }
    if(col[u]==1){f[u][1]=sum2,f[u][0]=0;return;}
    for(e=st[u];e!=-1;e=edge[e].nex)
    if((v=edge[e].to)!=pre)
    {
        exgcd(x,y,(f[v][0]+f[v][1])%mod,mod);
        sum1=(sum1+f[v][1]*(sum2*x%mod)%mod)%mod;
    }
    f[u][1]=sum1%mod,f[u][0]=sum2%mod;
}

int main()
{
    R int i,j;
    R int u,v;
    n=read();
    memset(st,-1,sizeof(st));
    et=0;
    for(i=1;i<=n;++i)col[i]=read();
    for(i=1;iread(),v=read();
        edge[et]=(ED){v,st[u]},st[u]=et++;
        edge[et]=(ED){u,st[v]},st[v]=et++;
    }
    dfs(1,1);
    printf("%lld",f[1][1]);
    return 0;
}