/in/foo.cc: In function 'void build(int, int)':
/in/foo.cc:10:2: error: 'lchild' was not declared in this scope
  lchild[loc] = idx + 1;
  ^~~~~~
/in/foo.cc:11:2: error: 'rchild' was not declared in this scope
  rchild[loc] = idx + 2;
  ^~~~~~