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;
}