Preorder Traversal

Overview

Preorder Traversal is a Tree DFS where the order is: Current Node, Left Child, Right Child. You process the parent before its children. It is heavily used when you want to copy or clone a tree, because you must create the parent node before you can attach children to it.

Syntax

Parents are fully processed before any deep dive begins.

Root, Left, Right

java

public void preorder(TreeNode root) {
    if (root == null) return;
    
    System.out.print(root.val); // 1. Root (Process)
    preorder(root.left);        // 2. Left
    preorder(root.right);       // 3. Right
}

Common Pitfalls

Using Preorder when you actually need sorted data (which requires Inorder).

Interview Tips

Preorder means the 'Root' is processed PRE (before) its children.

Real-World Example

Creating a completely deep copy of an existing Binary Tree.

example

java

public TreeNode clone(TreeNode root) {
    if (root == null) return null;
    TreeNode newNode = new TreeNode(root.val); // Preorder creation
    newNode.left = clone(root.left);
    newNode.right = clone(root.right);
    return newNode;
}

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