tree.c

#include <stdio.h>
#include <stdlib.h>

typedef struct _node node;

typedef void (*apply)(node *);

struct _node {
	struct _node *left;
	struct _node *right;
	char value;
};

void
insert(node **tree, char value) {
	node *n;
	if (!*tree) {
		n = malloc(sizeof(node));
		n->value = value;
		n->left  = NULL;
		n->right = NULL;
		*tree    = n;
		return;
	}
	
	if ((*tree)->value >= value)
		insert(&((*tree)->right), value);
	else
		insert(&((*tree)->left), value);
}

void
print(node *tree) {
	printf("%c\n", tree->value);
}

void
preorder_traversal(node *tree, apply f) {
	if (!tree) return;
	(*f)(tree);
	preorder_traversal(tree->left, f);
	preorder_traversal(tree->right, f);
}

void
postorder_traversal(node *tree, apply f) {
	if (!tree) return;
	postorder_traversal(tree->left, f);
	postorder_traversal(tree->right, f);	
	(*f)(tree);
}

void
inorder_traversal(node *tree, apply f) {
	if (!tree) return;
	inorder_traversal(tree->left, f);
	(*f)(tree);	
	inorder_traversal(tree->right, f);	
}

int
main(int argc, char *argv[]) {
	node* tree = NULL;

	insert(&tree, 'E');
	insert(&tree, 'F');
	insert(&tree, 'A');
	preorder_traversal(tree, print);
	printf("~~~~\n");	
	postorder_traversal(tree, print);
	printf("~~~~\n");
	inorder_traversal(tree, print);
	return 0;
}