View Compilation Error

0_0_28474524_31196.cpp:4:7: error: 'maxn' was not declared in this scope
 int a[maxn <<1];
       ^
0_0_28474524_31196.cpp: In function 'void build(int)':
0_0_28474524_31196.cpp:7:9: error: 'a' was not declared in this scope
     if(!a[1]){
         ^
0_0_28474524_31196.cpp: In function 'int main()':
0_0_28474524_31196.cpp:34:12: error: 'a' was not declared in this scope
         if(a[i] != 0)cout << a[i] <<" ";
            ^