Added min() and max() functions to tree

Added a min() function and a max() function to the tree. They are
simple (using recursion) and usefull so why not have them?

tree.c

@@ -108,3 +108,31 @@ void tree_treverse_inorder(struct tree *tree) {
 	tree_treverse_inorder_helper(tree->root);
 	printf("\n");
 }
+
+static void *tree_min_helper(struct node *node) {
+	if (node->left != NULL)
+		return tree_min_helper(node->left);
+	else
+		return node->data;
+}
+
+void *tree_min(struct tree *tree) {
+	if (tree->root != NULL)
+		return tree_min_helper(tree->root);
+	else
+		return NULL;
+}
+
+static void *tree_max_helper(struct node *node) {
+	if (node->right != NULL)
+		return tree_max_helper(node->right);
+	else
+		return node->data;
+}
+
+void *tree_max(struct tree *tree) {
+	if (tree->root != NULL)
+		return tree_max_helper(tree->root);
+	else
+		return NULL;
+}

tree.h

@@ -23,5 +23,8 @@ void tree_insert(struct tree *tree, void *data, size_t size);
 int tree_contains(struct tree *tree, void *data);
 void tree_dealloc(struct tree *tree);
 void tree_treverse_inorder(struct tree *tree);
+void *tree_min(struct tree *tree);
+void *tree_max(struct tree *tree);
+
 
 #endif /* TREE_H_ */