按顺序遍历二叉树到列表

Question

我需要为跟随树接口

创建顺序遍历的递归函数

// EFFECTS: returns true if tree is empty, false otherwise
bool tree_isEmpty (const tree_t& tree);
// EFFECTS: creates an empty tree.
tree_t tree_make ();
// EFFECTS: creates a new tree, with elt as it's element, left as
// its left subtree, and right as its right subtree
tree_t tree_make (int elt, const tree_t& left, const tree_t& right);
// REQUIRES: tree is not empty
// EFFECTS: returns the element at the top of tree.
int tree_elt (const tree_t& tree);
// REQUIRES: tree is not empty
// EFFECTS: returns the left subtree of tree
tree_t tree_left (const tree_t& tree);
// REQUIRES: tree is not empty
// EFFECTS: returns the right subtree of tree
tree_t tree_right (const tree_t& tree);
// MODIFIES: cout
// EFFECTS: prints tree to cout.
void tree_print (const tree_t& tree);

我不能用于，虽然或goto循环我尝试创建功能但不幸的是它不起作用请帮助。我的功能在下面

list_t traversal_helper(tree_t tree,list_t l){

    if (tree.is_empty())
        return l;

    /* first recur on left child */
    traversal_helper (tree.get_left_tree(),l);

    /* then print the data of node */
    l=list_make(tree.get_elt(),l);

    /* now recur on right child */
    traversal_helper (tree.get_right_tree(),l);
    }


list_t traversal(tree_t tree){
    list_t l=list_make();
    if(tree.is_empty()){
        return list_make();
    }
    else{
        return traversal_helper(tree,l);
    }
}

My list_t interface contain following functions:

List Interface
The file recursive.h defines the type "list_t" and the following operations on lists:
// EFFECTS: returns true if list is empty, false otherwise
bool list_isEmpty​ (const list_t& list);
// EFFECTS: returns an empty list.
list_t list_make​ ();
// EFFECTS: given the list (list) make a new list consisting of
// the new element followed by the elements of the
// original list.
list_t list_make​ (int elt, const list_t& list);
// REQUIRES: list is not empty
// EFFECTS: returns the first element of list
int list_first​ (const list_t& list);
// REQUIRES: list is not empty
// EFFECTS: returns the list containing all but the first element of list
list_t list_rest​ (const list_t& list);
// MODIFIES: cout
// EFFECTS: prints list to cout.
void list_print​ (const list_t& list);

Answer 1

traversal_helper永远不会返回任何内容，并且它不使用递归调用的返回值 - 我想你想要这样的东西：

list_t traversal_helper(tree_t tree,list_t l){

    if (tree.is_empty())
        return l;

    /* first recur on left child */
    l = traversal_helper (tree.get_left_tree(),l);

    /* then print the data of node */
    l=list_make(tree.get_elt(),l);

    /* now recur on right child */
    return traversal_helper (tree.get_right_tree(),l);
}

如果list_make创建一个在列表末尾附加元素的新列表，这应该有用。如果它将元素附加在列表的开头，那么结果将需要反转。