繁华模拟赛 Evensgn剪树枝

#include<iostream> 
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#include<cmath>
#define ll long long
using namespace std;
const int maxn = 100005;
const ll mod = 1000000007;
struct edge{
    int to;
    int nxt;
};
int n,f[maxn][2],col[maxn];
int cnt,head[maxn];
edge e[maxn];
void add_edge(int u,int v){
    cnt++;
    e[cnt].to = v;
    e[cnt].nxt = head[u];
    head[u] = cnt;
}
ll q_mul(ll a,ll b){
    ll ans = 0;
    while(b){
        if(b&1){
            ans = (ans + a) % mod;
        }
        a = (a + a) % mod;
        b >>= 1;
    }
    return ans;
}
ll q_pow(ll a,ll b){
    ll ans = 1;
    while(b){
        if(b&1){
            ans = q_mul(ans,a);
        }
        a = q_mul(a,a);
        b >>= 1;
    }
    return ans;
}
void input(){
    cin>>n;
    int cmd;
    for(int i = 1;i < n;i++){
        cin>>cmd;
        add_edge(cmd,i);
    }
    for(int i = 0;i < n;i++){
        cin>>col[i];
    }
}
void dfs(int u){
    if(!head[u]){
        if(col[u]) f[u][1] = 1;
        else f[u][0] = 1;
        return;
    }
    ll sum = 1;
    int v;
    if(col[u]){
        for(int i = head[u];i;i = e[i].nxt){
            v = e[i].to;
            dfs(v);
            sum = q_mul(sum,f[v][0] + f[v][1]);
        }
        f[u][1] = sum;
        return;
    }else{
        for(int i = head[u];i;i = e[i].nxt){
            v = e[i].to;
            dfs(v);
            sum = q_mul(sum,f[v][0] + f[v][1]);
        }
        f[u][0] = sum;
        for(int i = head[u];i;i = e[i].nxt){
            v = e[i].to;
            f[u][1] += q_mul(f[v][1],q_mul(sum,q_pow(f[v][0] + f[v][1],mod-2)));
        }
        return;
    }
}
int main(){
    freopen("tree.in","r",stdin);
    freopen("tree.out","w",stdout);
    ios::sync_with_stdio(false);
    input();
    dfs(0);
    cout<<f[0][1];
    return 0;
}



#include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <iostream>
using namespace std;

#define ll long long
const int MAXN = 1e5 + 10;
const int MOD  = 1e9 + 7;

ll f[MAXN][2];
int point[MAXN] = {0}, nxt[MAXN * 2] = {0}, v[MAXN * 2] = {0}, tot = 0;
bool color[MAXN] = {0};
int n;

inline void addedge(int x, int y)
{
    tot++;
    nxt[tot] = point[x]; point[x] = tot; v[tot] = y;
}

void dfs(int now, int father)
{
    f[now][0] = 1;
    f[now][1] = 0;
    for (int tmp = point[now]; tmp; tmp = nxt[tmp])
        if (v[tmp] != father)
        {
            dfs(v[tmp], now);
            f[now][1] = (f[now][1] * f[v[tmp]][0]) % MOD;
            f[now][1] = (f[now][1] + f[now][0] * f[v[tmp]][1]) % MOD;
            f[now][0] = (f[now][0] * f[v[tmp]][0]) % MOD;
        }
    if (color[now])
        f[now][1] = f[now][0];
    else
        f[now][0] = (f[now][0] + f[now][1]) % MOD;
}

int main()
{
    freopen("tree.in", "r", stdin);
    freopen("tree.out", "w", stdout);
    scanf("%d", &n);
    for (int i = 2; i <= n; i++)
    {
        int x;
        scanf("%d", &x);
        addedge(i, x + 1); addedge(x + 1, i);
    }
    for (int i = 1; i <= n; i++)
    {
        int x;
        scanf("%d", &x);
        if (x == 1) color[i] = true;
        else color[i] = false;
    }
    dfs(1, 0);
    cout << f[1][1] << endl;
}