提交时间:2026-04-22 17:02:14

运行 ID: 318264

#include <bits/stdc++.h> using namespace std; const int MOD = 1000000007; const int MAXN = 200005; long long dp[MAXN]; int main() { int n; cin >> n; dp[0] = 1; dp[1] = 3; int k = n / 2; for (int i = 2; i <= k; ++i) { dp[i] = (4 * dp[i-1] % MOD - dp[i-2] + MOD) % MOD; } cout << dp[k] << endl; return 0; }