~~高精度+递推很难吗？~~
```cpp
#include<bits/stdc++.h>
using namespace std;
//f[i]=f[i-1]*2+2
int n;
int a[1000];
int main(){
cin >> n;
a[1] = 2;
for(int i = 2; i <= n; i++){
for(int j = 1; j <= 500; j++){
a[j] *= 2;
}
a[1] += 2;
for(int j = 1; j <= 500; j++){
a[j + 1] += a[j] / 10;
a[j] %= 10;
}
}
int len;
for(len = 500; a[len] == 0; len--);
for(int i = len; i >= 1; i--){
cout << a[i];
}
return 0;
}
```
by 1_1_1_1_1_1_ @ 2023-12-17 13:36:59
@[XZWUL](/user/1088906)
没必要定义char类型的数组
by 1_1_1_1_1_1_ @ 2023-12-17 13:38:21
不是直接研究通项啊
by Change_YuAN @ 2024-02-21 20:24:28