P4451 [国家集训队]整数的lqp拆分

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
inline LL read () { LL res = 0 ;int f (1) ;char ch = getchar ();
    while (!isdigit(ch)) { if (ch == '-') f = -1 ;ch = getchar();}
    while (isdigit(ch)) res = (res << 1) + (res << 3) + (ch ^ 48) ,ch = getchar(); return res * f ;
}
LL n;
LL a[1<<20];
signed main () {
    n=read();
    a[1]=1; a[2]=2;
    for(register int i=3;i<=n;i++) a[i]=((a[i - 1] << 1)+(a[i - 2])) %1000000007 ;
    cout << a[n] << endl ;
    return 0;
}