Traversing a general tree recursively in depth-first order - c

Question

I am asked to implement a recursive function for traversing a general tree (first child, next sibling notation) and a binary tree in depth-first order.
The function should print the node when it last visits it. For example, for the tree below, the nodes should be printed in the following order.
I think I have done the function for the binary tree but I could not do the general one:
Here is my code for the binary tree:
void PostOrder(node* root) {
if (root == NULL)
return;
else {
PostOrder(root->left);
PostOrder(root->right);
printf(‘%c’, root->key);
}
}
Can anyone help?

Answer 1

It sounds like it's a typical "general tree represented as binary tree" with two pointers: first child, and next sibling. We can draw the tree as a standard binary tree, with each child having a down-leading arrow and a right-leading arrow.
I think the tree described in your comment looks like this:
a
|
V
b ------------> c
| |
V V
d --> e --> f g
|
V
h --> i
You want, at each node of the tree, to:
Print all the descendants first, by processing the first child (follow the down arrow).
Then print the node's value
Then process the siblings (follow the right arrow).
Applying this to the tree above, we get:
GeneralTreePostOrder(a) ->
GeneralTreePostOrder(b) -> # Follow child pointer
GeneralTreePostOrder(d) -> # Follow child pointer
# No children
print d
GeneralTreePostOrder(e) -> # Follow sibling pointer
# No children
print e
GeneralTreePostOrder(f) -> # Follow sibling pointer
# No children
print f
# No more siblings
print b
GeneralTreePostOrder(c) -> # Follow sibling pointer
Etc., which prints d e f h i g c a.
Writing a new function for the general tree (N.B. printf should use a double-quoted format string):
void GeneralTreePostOrder (node* root) {
if (root == NULL)
return;
else {
GeneralTreePostOrder (root->left); // Process children
printf ("%c", root->key);
GeneralTreePostOrder (root->right); // Process siblings
}
}