Codeforces Round #263 (Div. 1)

 1 #include <iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<stdlib.h>
 6 #include<vector>
 7 #include<cmath>
 8 #include<queue>
 9 #include<set>
10 using namespace std;
11 #define N 100010
12 #define LL long long
13 #define INF 0xfffffff
14 #define mod 1000000007
15 const double eps = 1e-8;
16 const double pi = acos(-1.0);
17 const double inf = ~0u>>2;
18 vector<int>ch[N];
19 int dp[N][2];
20 int co[N];
21 LL q_mod(LL a,LL b)
22 {
23     LL d,t;
24     d = 1,t=a;
25     while(b)
26     {
27         if(b&1) d = (d*t)%mod;
28         b/=2;
29         t = (t*t)%mod;
30     }
31     return d;
32 }
33 LL cal(LL s,LL y,LL x)
34 {
35     s = (s*x)%mod;
36     return (s*q_mod(y,mod-2))%mod;
37 }
38 void dfs(int u,int pre)
39 {
40     int i;
41     LL s0=1;
42     int flag = 0;
43     for(i = 0 ; i < ch[u].size() ;i++)
44     {
45         int v = ch[u][i];
46         if(v==pre) continue;
47         flag = 1;
48         dfs(v,u);
49         s0 = (s0*dp[v][0])%mod;
50     }
51     if(!flag)
52     {
53         dp[u][0] = 1;
54         dp[u][1] = co[u];
55         return ;
56     }
57     if(co[u]==0)
58     {
59         dp[u][0] = s0;
60         dp[u][1] = 0;
61         for(i = 0 ;i < ch[u].size() ; i++)
62         {
63             int v = ch[u][i];
64             if(v==pre) continue;
65             if(!dp[v][1]) continue;
66             dp[u][1] = (dp[u][1]+cal(s0,dp[v][0],dp[v][1]))%mod;
67         }
68         dp[u][0] = (dp[u][0]+dp[u][1])%mod;
69     }
70     else
71     {
72         dp[u][0] = s0;
73         dp[u][1] = s0;
74     }
75 }
76 int main()
77 {
78     int n,i;
79     cin>>n;
80     for(i = 0 ; i < n-1; i++)
81     {
82         int u;
83         scanf("%d",&u);
84         ch[u].push_back(i+1);
85         ch[i+1].push_back(u);
86     }
87     for(i = 0 ; i < n; i++)
88     scanf("%d",&co[i]);
89     dfs(0,-1);
90     cout<<dp[0][1]<<endl;
91     return 0;
92 }