In file included from /in/ex6.cpp:1:0:
/in/ex6.h:137:3: error: 'string' does not name a type; did you mean 'stdin'?
   string inOrderTraversal_helper(TreeNode *root) const;
   ^~~~~~
   stdin
/in/ex6.cpp: In member function 'void BinaryTree::inOrderTraversal() const':
/in/ex6.cpp:47:16: error: 'inOrderTraversal_helper' was not declared in this scope
     string s = inOrderTraversal_helper(this->root);
                ^~~~~~~~~~~~~~~~~~~~~~~
/in/ex6.cpp:47:16: note: suggested alternative: 'inOrderTraversal'
     string s = inOrderTraversal_helper(this->root);
                ^~~~~~~~~~~~~~~~~~~~~~~
                inOrderTraversal
/in/ex6.cpp: At global scope:
/in/ex6.cpp:96:60: error: no 'std::__cxx11::string BinaryTree::inOrderTraversal_helper(TreeNode*) const' member function declared in class 'BinaryTree'
 string BinaryTree::inOrderTraversal_helper(TreeNode *root) const{
                                                            ^~~~~
/in/compile/Makefile:24: recipe for target 'ex6.o' failed
make: *** [ex6.o] Error 1