CodeForces 416 B Appleman and Tree DP

#include<bits/stdc++.h>
using namespace std;
#define Fopen freopen("_in.txt","r",stdin); freopen("_out.txt","w",stdout);
#define LL long long
#define ULL unsigned LL
#define fi first
#define se second
#define pb push_back
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define lch(x) tr[x].son[0]
#define rch(x) tr[x].son[1]
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
typedef pair<int,int> pll;
const int inf = 0x3f3f3f3f;
const int _inf = 0xc0c0c0c0;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const LL _INF = 0xc0c0c0c0c0c0c0c0;
const LL mod =  (int)1e9+7;
const int N = 1e5 + 100;
vector<int> vc[N];
int dp[N][2];
int pre[N], suf[N];
int a[N];
void dfs(int u){
    if(a[u] == 1) {
        dp[u][1] = 1; dp[u][0] = 0;
        for(int v : vc[u]){
            dfs(v);
            dp[u][1] = (1ll * dp[u][1] * (dp[v][1] + dp[v][0])) % mod;
        }
    }
    else{
        dp[u][1] = 0; dp[u][0] = 1;
        int t = vc[u].size();
        for(int v : vc[u]){
            dfs(v);
        }
        for(int i = 0; i < t; ++i){
            int v = vc[u][i];
            suf[i+1] = pre[i+1] = (dp[v][0]+dp[v][1]) % mod;
        }
        pre[0] = 1; suf[t+1] = 1;
        for(int i = 1; i <= t; ++i)
            pre[i] = 1ll * pre[i] * pre[i-1] % mod;
        for(int i = t; i >= 1; --i)
            suf[i] = 1ll * suf[i] * suf[i+1] % mod;
        dp[u][0] = pre[t];
        for(int i = 1; i <= t; ++i){
            int v = vc[u][i-1];
            dp[u][1] = (dp[u][1] + 1ll * dp[v][1] * (1ll * pre[i-1] * suf[i+1]%mod))% mod;
        }
    }
}
int main(){
    int n, o;
    scanf("%d", &n);
    for(int i = 2; i <= n; ++i){
        scanf("%d", &o);
        vc[o+1].pb(i);
    }
    for(int i = 1; i <= n; ++i)
        scanf("%d", &a[i]);
    dfs(1);
    printf("%d\n", dp[1][1]);
    return 0;
}