View Code of Problem 3692

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cctype>
#include<vector>
#include<cmath>
using namespace std;
int main(void){
	a[1] = 3;
    a[2] = 6;
    a[3] = 6;
    int n;
    for (int i = 4; i <= n; i++)
    {
        a[i] = a[i - 1] + a[i - 2] * 2;
    }
    while (cin >> n)
    {
        cout << a[n] << endl;
    }
} 

/*
Main.cc: In function 'int main()':
Main.cc:10:2: error: 'a' was not declared in this scope
  a[1] = 3;
  ^
*/