Problem:
Score : 1000 points

Problem Statement

Inspired by the tv series Stranger Things, bear Limak is going for a walk between two mirror worlds.

There are two perfect binary trees of height H,
each with the standard numeration of vertices from 1 to 2^H-1.
The root is 1 and the children of x are 2 \cdot x and 2 \cdot x + 1.

Let L denote the number of leaves in a single tree, L = 2^{H-1}.

You are given a permutation P_1, P_2, \ldots, P_L of numbers 1 through L.
It describes L special edges that connect leaves of the two trees.
There is a special edge between vertex L+i-1 in the first tree and vertex L+P_i-1 in the second tree.

drawing for the first sample test, permutation P = (2, 3, 1, 4), special edges in green

Let's define product of a cycle as the product of numbers in its vertices.
Compute the sum of products of all simple cycles that have exactly two special edges, modulo (10^9+7).

A simple cycle is a cycle of length at least 3, without repeated vertices or edges.

Constraints

2 \leq H \leq 18

1 \leq P_i \leq L where L = 2^{H-1}

P_i \neq P_j (so this is a permutation)

Input

Input is given from Standard Input in the following format (where L = 2^{H-1}).

H
P_1 P_2 \cdots P_L

Output

Compute the sum of products of simple cycles that have exactly two special edges.
Print the answer modulo (10^9+7).

Sample Input 1

3
2 3 1 4

Sample Output 1

121788

The following drawings show two valid cycles (but there are more!) with products
2 \cdot 5 \cdot 6 \cdot 3 \cdot 1 \cdot 2 \cdot 5 \cdot 4 = 7200
and 1 \cdot 3 \cdot 7 \cdot 7 \cdot 3 \cdot 1 \cdot 2 \cdot 5 \cdot 4 \cdot 2 = 35280.
The third cycle is invalid because it doesn't have exactly two special edges.

Sample Input 2

2
1 2

Sample Output 2

36

The only simple cycle in the graph indeed has two special edges, and the product of vertices is
1 \cdot 3 \cdot 3 \cdot 1 \cdot 2 \cdot 2 = 36.

Sample Input 3

5
6 14 15 7 12 16 5 4 11 9 3 10 8 2 13 1

Sample Output 3

10199246

Solution in C++ (GCC 9.2.1) (judge verdict: Time Limit Exceeded)

#include <bits/stdc++.h>
using namespace std;

using ll=long long;
#define rng(i,l,r) for(int i=int(l);i<int(r);i++)
#define rep(i,r) rng(i,0,r)
#define rrng(i,l,r) for(int i=int(r)-1;i>=int(l);i--)
#define rrep(i,r) rrng(i,0,r)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define F first
#define S second
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
#define si(x) int(x.size())
#define inf INT_MAX/2-100
#define infl LLONG_MAX/3
#ifdef LOCAL
#define dmp(x) cerr<<__LINE__<<' '<<#x<<' '<<x<<endl
#else
#define dmp(x) void(0)
#endif

template<class t,class u>bool chmax(t&a,u b){if(a<b)a=b;return a<b;}
template<class t,class u>bool chmin(t&a,u b){if(b<a)a=b;return b<a;}

template<class t>using vc=vector<t>;
template<class t>using vvc=vector<vector<t>>;

using pi=pair<int,int>;
using pl=pair<ll,ll>;
using vi=vc<int>;
using vl=vc<ll>;


ll readl(void){
	ll x;
	cin>>x;
	return x;
}
int readi(void){
	int x;
	cin>>x;
	return x;
}

string readstr(){
	string s;
	cin>>s;
	return s;
}
vi readvi(int n,int off=0){
	vi v(n);
	rep(i,n)v[i]=readi(),v[i]+=off;
	return v;
}

vl readvl(int n,int off=0){
	vl v(n);
	rep(i,n)v[i]=readl(),v[i]+=off;
	return v;
}

template<class t>
void print(t x,int suc=1){
	cout<<x;
	if(suc==1)cout<<"\n";
	if(suc==2)cout<<" ";
}

template<class t>
void print(const vc<t>&v,int suc=1){
	rep(i,si(v))print(v[i],i==int(si(v))-1?1:suc);
}

template<class t>
bool inc(t a,t b,t c){
	return !(c<b||b<a);
}

template<class t>
void compress(vc<t>&v){
	sort(all(v));
	v.erase(unique(all(v)),v.ed);
}

template<class t>
int lwb(const vc<t>&v,const t&a){
	return lower_bound(all(v),a)-v.bg;
}

template<class t>
struct Compress{
	vc<t>v;
	Compress()=default;
	Compress(const vc<t>&x){
		add(x);
	}
	Compress(const initializer_list<vc<t> >&x){
		for(auto &p:x)add(p);
	}
	void add(const t&x){
		v.eb(x);
	}
	void add(const vc<t>&x){
		copy(all(x),back_inserter(v));
	}
	void build(){
		compress(v);
	}
	int get(const t&x)const{
		return lwb(v,x);
	}
	vc<t>get(const vc<t>&x)const{
		vc<t>res(x);
		for(auto &p:res)p=get(p);
		return res;
	}
	const t &operator[](int x)const{
		return v[x];
	}
	int size(){
		return v.size();
	}
};
void Yes(bool ex=true){
	cout<<"Yes\n";
	if(ex)exit(0);
}
void YES(bool ex=true){
	cout<<"YES\n";
	if(ex)exit(0);
}
void No(bool ex=true){
	cout<<"No\n";
	if(ex)exit(0);
}
void NO(bool ex=true){
	cout<<"NO\n";
	if(ex)exit(0);
}
void orYes(bool x,bool ex=true){
	if(x)Yes(ex);
	else No(ex);
}
void orYES(bool x,bool ex=true){
	if(x)YES(ex);
	else NO(ex);
}
void Possible(bool ex=true){
	cout<<"Possible\n";
	if(ex)exit(0);
}
void POSSIBLE(bool ex=true){
	cout<<"POSSIBLE\n";
	if(ex)exit(0);
}
void Impossible(bool ex=true){
	cout<<"Impossible\n";
	if(ex)exit(0);
}
void IMPOSSIBLE(bool ex=true){
	cout<<"IMPOSSIBLE\n";
	if(ex)exit(0);
}
void orPossible(bool x,bool ex=true){
	if(x)Possible(ex);
	else Impossible(ex);
}
void orPOSSIBLE(bool x,bool ex=true){
	if(x)POSSIBLE(ex);
	else IMPOSSIBLE(ex);
}

using uint=unsigned;
using ull=unsigned long long; 

template<uint const& MOD>
struct Modular{
	static constexpr uint const &mod=MOD;
	uint v;
	Modular(long long x=0){c(x%mod+mod);}
	Modular& c(uint x){
		v=x<mod?x:x-mod;
		return *this;
	}
	Modular pow(int k)const{
		Modular res(1),tmp(v);
		while(k){
			if(k&1)res*=tmp;
			tmp*=tmp;
			k>>=1;
		}
		return res;
	}

	Modular inv()const{return pow(mod-2);}
	Modular operator-()const{return Modular(mod-v);}

	Modular& operator+=(const Modular &x){return c(v+x.v);}
	Modular& operator-=(const Modular &x){return c(v+mod-x.v);}
	Modular& operator*=(const Modular &x){v=ull(v)*x.v%mod;return *this;}
	Modular& operator/=(const Modular &x){return *this*=x.inv();}

	Modular operator+(const Modular &x)const{return Modular(*this)+=x;}
	Modular operator-(const Modular &x)const{return Modular(*this)-=x;}
	Modular operator*(const Modular &x)const{return Modular(*this)*=x;}
	Modular operator/(const Modular &x)const{return Modular(*this)/=x;}

	friend Modular operator+(long long x,const Modular &y){return Modular(x)+y;}
	friend Modular operator-(long long x,const Modular &y){return Modular(x)-y;}
	friend Modular operator*(long long x,const Modular &y){return Modular(x)*y;}
	friend Modular operator/(long long x,const Modular &y){return Modular(x)/y;}

	friend ostream& operator<<(ostream&os,const Modular&x){
		return os<<x.v;
	}
	friend istream& operator>>(istream&is,Modular&x){
		long long p;
		is>>p;
		x=Modular(p);
		return is;
	}
	bool operator==(const Modular &x)const{return v==x.v;}
	bool operator!=(const Modular &x)const{return v!=x.v;}
	bool operator<(const Modular &x)const{return v<x.v;}
	explicit operator bool()const{return v;}
};

//uint MODULAR=998244353;
uint MODULAR=1000000007;
using Mint=Modular<MODULAR>; 

vector<Mint>fact,finv,invs;
void Initfact(int n=(1<<21)+10){
	fact.resize(n+1),finv.resize(n+1),invs.resize(n+1);
	fact[0]=1;
	for(int i=1;i<=n;i++){
		fact[i]=fact[i-1]*i;
	}
	finv[n]=fact[n].inv();
	for(int i=n-1;i>=0;i--){
		finv[i]=finv[i+1]*(i+1);
	}
	invs[0]=1;
	for(int i=0;i<=n;i++){
		invs[i]=finv[i]*fact[i-1];
	}
}

Mint comb(int n,int k){
	return fact[n]*finv[n-k]*finv[k];
}

int h;
Mint r[(1<<18)+1],invr[(1<<18)+1],ans;
map<int,Mint>sum[(1<<18)+1][2];
int main(void){
	cin.tie(0);
	ios::sync_with_stdio(0);
	cin>>h;
	for(int i=1;i<1<<h;i++){
		r[i]=i;
		if(i/2>0)r[i]*=r[i/2];
		invr[i]=r[i].inv()*r[i].inv()*Mint(i);
	}
	for(int i=0;i<1<<(h-1);i++){
		int p;
		cin>>p;
		p--;
		p+=(1<<(h-1));
		int v=i+(1<<(h-1));
		Mint q=r[p]*r[v];
		while(v>1){
			sum[p][v%2][v/2]+=q;
			v/=2;
		}
	}
	for(int i=(1<<(h-1))-1;i>0;i--){
		for(int j=0;j<2;j++){
			for(auto &k:sum[i*2][j]){
				ans+=invr[i]*invr[k.F]*k.S*sum[i*2+1][(j+1)%2][k.F];
				sum[i][j][k.F]+=k.S;
			}
			for(auto &k:sum[i*2+1][j]){
				sum[i][j][k.F]+=k.S;
			}
		}
	}
	print(ans);
}