quartz/content/notes/tree-traversal.md

title	aliases	tags
tree-traversal	traversal	cosc201

Visit each node in the tree once. So we need to visit n, all the nodes in L, and all the nodes in R. We can do this in four different ways

• Preorder: Root, left, right
• Postorder: Left, right, root
• Inorder: Left, Root Right (produces ordered list in a BST)
• Level order: root, all the roots children, all their children, etc

examples

Creating an BST from an array using the Add Operation operation then doing an inorder traversal is effectively a quicksort

Code

Usually traversals ideas used in algorithms, not independent methods

Pre order

private void preorder(Node n, ArrayList<String> r){
	if(n==null) return;
	r.add(n.key); //add this node the reults
	preorder(n.left, r); //traverse the left subtree
	preorder(r.right, r); //traverse the right subtree
}

Post order

private void postorder(Node n, ArrayList<String> r){
	if(n==null) return;
	postorder(n.left, r); //traverse the left subtree
	postorder(r.right, r); //traverse the right subtree
	r.add(n.key); //add this node the reults
}

In order

private void inorder(Node n, ArrayList<String> r){
	if(n==null) return;
	inorder(n.left, r); //traverse the left subtree
	r.add(n.key); //add this node the reults
	inorder(r.right, r); //traverse the right subtree
}

Level order

Only traversal that is not naturally recursive. Maintains a queue of pending visits. Using a stack with this methods produces a preorder

public Arraylist<String> levelorder() {
	ArrayList<String> result = new ArrayList<>();
	if(isEmpty()) return result;
	ArrayDeque<Node> q = new ArrayDeque<>();
	q.add(root)
	while (!q.isEmpty()) {
		Node n = q.remove()
		result.add(n.key);
		if(n.left != null) q.add(n.left);
		if(n.right != null) q.add(n.right);
	}
	return result;
}