题解 | #小红的二叉树#

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#include <math.h>
#include<bits/stdc++.h>

using namespace std;

#define int long long

const int MAXN=10000000000000000;
const int MOD=1e9+7;
const int N=1000100;

struct Tree{
    int l,r,sum,add;
}tree[N];

void build(int p,int l,int r) {//递归建树
    if(l>r) return ;

    tree[p].l=l;
    tree[p].r=r;
    int mid = (l+r)>>1;
    build(p*2,l,mid);
    build(p*2+1,mid+1,r);
    tree[p].sum = tree[p*2].sum + tree[p*2+1].sum;

    return ;
}

int qpow(int num,int k) {
    int res=1;
    while(k) {
        if(k%2) res = (res * num) % MOD;
        num = (num * num) % MOD;
        k/=2;
    }
    return res%MOD;
}

void init() {

}

int a[N];
int b[N];

void solve() {
    init();

    int n;
    cin>>n;
    if(n==1) {
        cout<<0<<"\n";
        return ;
    }
    else if(n==2) {
        cout<<1<<"\n";
        return ;
    }
    int leaf = qpow(2,n-1);
    cout<<( ( qpow(2,n)-1 - leaf) % MOD + 4 * ( ( 1 - qpow(2,n-2) ) ) * (-1) ) % MOD<<"\n";
}


signed main() {
     //ios::sync_with_stdio(false);
     //cin.tie(0);
     //cout.tie(0);

    int t=1;
    //cin>>t;

    while(t--) {
        solve();
    }
    return 0;
}