时间复杂度 O(n)空间复杂度 O(1)

#include <cstdio>
int main()
{
int n;
scanf("%d", &n);
long long a = 2, b = 7;
if (n == 1) { printf("2\n"); return 0; }
if (n == 2) { printf("7\n"); return 0; }
for (int i = 3; i <= n; ++i)
{
long long c = 2 * b + 2 * a + 3;
a = b;
b = c;
}
printf("%lld\n", b);
return 0;
}