Submission #23367542

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Source Code Expand

#include<bits/stdc++.h>

#define ll long long 

using namespace std;

int read()
{
	int a=0,f=1,c=getchar();
	while(!isdigit(c))
	{
		if(c=='-')f=-1;
		c=getchar();
	}
	while(isdigit(c))
	{
		a=a*10+c-'0';
		c=getchar();
	}
	return a*f;
}

const int N=1e5+10;
const int P=1e9+7;
ll mod(ll x){return (x%P+P)%P;}

int n;
int a[N];
ll f[2][N],g[2][N];

int main()
{
//	freopen("1.in","r",stdin);
	
	n=read();
	for(int i=1;i<=n;++i)a[i]=read();
	
	f[0][1]=1;
	g[0][1]=a[1];
	for(int i=2;i<=n;++i)
	{
		f[0][i]=mod(f[0][i-1]+f[1][i-1]);
		f[1][i]=f[0][i-1];
		g[0][i]=mod(g[0][i-1]+g[1][i-1]+a[i]*f[0][i]);
		g[1][i]=mod(g[0][i-1]-a[i]*f[1][i]);
	}
	printf("%lld\n",mod(g[0][n]+g[1][n]));
	
	return 0;
}

Submission Info

Judge Result