What is the Types of DFS (Tree Traversals) pattern?

The Types of DFS (Tree Traversals) is a common coding interview pattern in Depth First Search problems. It provides a structured approach to solving related algorithm challenges efficiently.

When should I use Types of DFS (Tree Traversals)?

Use the Types of DFS (Tree Traversals) pattern when you encounter problems matching its characteristics. Check the "When to Use" and "Clues" sections in the lesson for specific indicators.

Is Types of DFS (Tree Traversals) asked in FAANG interviews?

Yes, Types of DFS (Tree Traversals) is an essential pattern commonly tested in FAANG and top tech company technical interviews.

Depth-First Search

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Types of DFS (Tree Traversals)

Master Preorder, Inorder, and Postorder traversals: the building blocks of most Tree algorithms.

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The Three Flavors of DFS

When traversing a Tree using Depth-First Search, the "depth-first" logic remains the same (go down before going sideways). However, the order in which we "process" (visit/print) the current node relative to its children defines the three main types:

Preorder: Root → Left → Right
Inorder: Left → Root → Right
Postorder: Left → Right → Root

Problem: Standardize the Visit

Given a binary tree, return a list of values depending on the traversal order.

Input: Tree [1, null, 2, 3] (1 is root, 2 is right child, 3 is left child of 2)
Preorder Output: [1, 2, 3]
Inorder Output: [1, 3, 2]
Postorder Output: [3, 2, 1]

When to use each type?

Pattern Recognition:

Preorder: Creating a copy of the tree, Prefix expression evaluation, Serialization.
Inorder: Binary Search Tree (BST) validation (gives sorted order), Infix expression.
Postorder: Deleting a tree (delete children before parent), Postfix evaluation, Computing height/size (need child info first).

Intuition: The Tour Route

Imagine tracing the outline of the tree starting from the left of the root.

Preorder: Snap a photo the moment you arrive at a node for the first time.
Inorder: Snap a photo only after you've returned from the left child but before going right.
Postorder: Snap a photo only when you are leaving the node for the last time (finished both children).

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Iterative Approaches (Stack)

Using the call stack (recursion) is elegant, but sometimes we need to simulate it manually using an explicit stack.

Preorder: Push Root. While stack: Pop, process, push Right, push Left.
Inorder: Push all left nodes. Pop, process, move Right, repeat.
Postorder: Tricky (Use 2 stacks or reverse Preorder Root-Right-Left).

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