题解 | 小红蹦跳蹦跳

#include <algorithm>
#include <iostream>
#include <vector>
#define rep(i,x,y) for(int i=x;i<=y;i++)
using namespace std;
using ll = long long;
constexpr ll mod = 1e9+7;
int main() {
    int n; cin >> n;
    vector<ll> a(n+5);
    auto f = vector(n+5, vector(2, 0));
    auto s = vector(n+5, vector(2, vector(2, 0)));
    rep(i,1,n) cin >> a[i];
    rep(i,1,n) {
        s[i] = s[i-1];
        rep(j,0,1) {
            f[i][j] = s[i-1][!j][(i % 2) ^ j] + (j == i % 2);
            s[i][j][i % 2] += f[i][j];
            s[i][j][i % 2] %= mod;
        }
    }
    cout << (f[n][0] + f[n][1]) % mod << endl;
}