// lanqiao 2110 (积木画)

13lanqiao/test7.cpp

@@ -0,0 +1,22 @@
+// lanqiao 2110 (积木画)
+#include<bits/stdc++.h>
+using namespace std;
+const int N = 10000010, MOD = 1e9 + 7;
+int f[N];
+/*
+递推法:
+f1=1 f2=2 f3=5 f4=11 f5=24 f6=53
+f[n] = 2*f[n-1] + f[n-3]
+*/
+int main(){
+    int n; cin >> n;
+    f[1] = 1, f[2] = 2; f[3] = 5;
+    for(int i = 4; i <= n; i++){
+        f[i] = (2*f[n-1] % MOD + f[i-3] % MOD) % MOD;
+    }
+    cout << f[n] << endl;
+    return 0;
+}
+/* test samples -> 5
+3
+*/