/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:23:10: error: 'a' was not declared in this scope
  cout << a[cur].x;
          ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:31:10: error: 'a' was not declared in this scope
   cin >> a[i];
          ^
/in/foo.cc:33:16: error: 'a' was not declared in this scope
  tr[++sum].x = a[1];
                ^