Submission #1863358

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Source Code Expand

#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <queue>
#include <algorithm>
#include <utility>
#include <cmath>
#include <map>
#include <set>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define INF_LL (ll)1e18
#define INF (int)1e9
#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(x) x.begin(),x.end()
using ll = long long;
using PII = pair<int, int>;

const double eps = 1e-10;

template<typename A, typename B>inline void chmin(A &a, B b){if(a > b) a = b;}
template<typename A, typename B>inline void chmax(A &a, B b){if(a < b) a = b;}

const ll mod = 1e9+7;

vector<int> G[214514];

ll mpow(ll a, ll b){
	if(b == 0) return 1;
	if(b < 0) return 0;
	if(b%2) return (a*mpow(a, b-1))%mod;
	ll ret = mpow(a, b/2);
	return (ret*ret)%mod;
}

ll dp[2010][2] = {};
ll cd[214514] = {};

void init(int v, int dd){
	cd[dd]++;
	REP(i, G[v].size()){
		init(G[v][i], dd+1);
	}
}

void dfs(int v, int d){
	if(d == 0){
		dp[v][0] = dp[v][1] = 1;
		return;
	}
	ll mul = 1, mul2 = 1;;
	REP(i, G[v].size()){
		dfs(G[v][i], d-1);
		mul *= dp[G[v][i]][0];
		mul %= mod;
		mul2 = (mul2 * (dp[G[v][i]][0]+dp[G[v][i]][1]))%mod;
	}
	REP(i, G[v].size()){
		dp[v][1] += ((mul*mpow(dp[G[v][i]][0], mod-2))%mod*dp[G[v][i]][1])%mod;
		dp[v][1] %= mod;
	}
	dp[v][0] = (mul2-dp[v][1]+mod)%mod;
}

int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N;
	cin >> N;
	if(N > 2000) return 0;
	REP(i, N){
		int p;
		cin >> p;
		G[p].push_back(i+1);
	}
	init(0, 0);
	ll res= 0;
	REP(i, N+1){
		REP(j, 2010) REP(k, 2)
			dp[j][k] = 0;
		dfs(0, i);
		res = (res+dp[0][1]*mpow(2, N-cd[i]+1))%mod;
	}
	cout << res << endl;
}

Submission Info

Judge Result