How is a binary tree traversed using pre-order traversal?

In pre-order traversal, a binary tree is traversed in the order of root, left subtree, then right subtree.

Pre-order traversal is a type of depth-first traversal method used for binary trees in computer science. The name 'pre-order' comes from the fact that the root node of the tree is processed before (pre) the subtrees. The process is recursive in nature and follows a specific order: first, the root node is visited, then the left subtree, and finally the right subtree.

To start, you visit the root node of the binary tree. This is the 'pre' part of the pre-order traversal. You then move to the left subtree and repeat the process. You visit the root of the left subtree (which is now considered as the 'current' root), then its left subtree, and so on. This continues until you reach a node with no left child, at which point you begin visiting the right child.

Once the left subtree is completely visited, the process is repeated for the right subtree. You visit the root of the right subtree, then its left subtree, and so on. If a node has no children (i.e., it is a leaf node), you simply visit that node.

This method of traversal is particularly useful when you need to create a copy of the tree or to get a prefix expression on a binary tree. Also, pre-order traversal is recursive in nature, with the base case being when a null subtree is hit.

In terms of implementation, pre-order traversal can be done both iteratively and recursively. The recursive method is more straightforward: a function is called on the root, which then calls itself on the left child, then on the right child. The iterative method involves the use of a stack to keep track of nodes.

In summary, pre-order traversal is a systematic way of visiting all the nodes in a binary tree, where the root node is always visited before its subtrees. It's a fundamental concept in tree-based data structures, with applications in various computer science problems.