Tree Traversal Preorder Root Left Right in DSA C# - Build from Scratch

Tree Traversal Preorder Root Left Right

📖 Scenario: Imagine you have a family tree, and you want to visit each family member starting from the oldest ancestor, then moving to their children from left to right.

🎯 Goal: You will build a simple tree structure and write code to visit each node in preorder: first the root, then the left child, then the right child.

📋 What You'll Learn

Create a class called Node with an integer Value and two children: Left and Right

Create a tree with exactly three nodes: root with value 1, left child with value 2, right child with value 3

Write a method called PreorderTraversal that visits nodes in preorder and collects their values in a list

Print the list of visited node values after traversal

💡 Why This Matters

🌍 Real World

Tree traversal is used in many areas like file system navigation, expression evaluation, and organizing data hierarchically.

💼 Career

Understanding tree traversal is important for software developers working with data structures, algorithms, and system design.

1

Create the Node class and the tree root

Create a class called Node with an integer property Value and two properties Left and Right of type Node. Then create a variable called root and assign it a new Node with Value 1.

DSA C#

// Define the Node class and create the root node with Value 1
// Your code here

Hint

Define the Node class with a constructor that sets the Value. Then create root as a new Node with value 1.

2

Add left and right children to the root

Add a left child to root with Value 2 and a right child with Value 3 by assigning new Node instances to root.Left and root.Right.

DSA C#

public class Node
{
    public int Value;
    public Node Left;
    public Node Right;

    public Node(int value)
    {
        Value = value;
        Left = null;
        Right = null;
    }
}

Node root = new Node(1);
// Add left child with Value 2 and right child with Value 3 to root
// Your code here

Hint

Use root.Left = new Node(2); and root.Right = new Node(3); to add children.

3

Write the PreorderTraversal method

Write a method called PreorderTraversal that takes a Node and a List<int> called result. It should add the current node's Value to result, then recursively call itself on the Left child, then on the Right child.

DSA C#

public class Node
{
    public int Value;
    public Node Left;
    public Node Right;

    public Node(int value)
    {
        Value = value;
        Left = null;
        Right = null;
    }
}

Node root = new Node(1);
root.Left = new Node(2);
root.Right = new Node(3);

// Write PreorderTraversal method below
// Your code here

Hint

Check if node is null first. Then add node.Value to result. Then call PreorderTraversal on node.Left and node.Right.

4

Call PreorderTraversal and print the result

Create a List<int> called visited. Call PreorderTraversal with root and visited. Then print the values in visited separated by spaces using Console.WriteLine(string.Join(" ", visited)).

DSA C#

using System.Collections.Generic;

public class Node
{
    public int Value;
    public Node Left;
    public Node Right;

    public Node(int value)
    {
        Value = value;
        Left = null;
        Right = null;
    }
}

Node root = new Node(1);
root.Left = new Node(2);
root.Right = new Node(3);

void PreorderTraversal(Node node, List result)
{
    if (node == null) return;
    result.Add(node.Value);
    PreorderTraversal(node.Left, result);
    PreorderTraversal(node.Right, result);
}

// Create visited list, call PreorderTraversal, and print the result
// Your code here

Hint

Create a list called visited, call PreorderTraversal(root, visited), then print with Console.WriteLine(string.Join(" ", visited)).

Practice

1. What is the order of nodes visited in a preorder tree traversal?

easy

A. Left, Root, Right

B. Right, Left, Root

C. Left, Right, Root

D. Root, Left, Right

Solution

1. Step 1: Understand preorder traversal definition

   Preorder traversal visits the root node first, then recursively visits the left subtree, followed by the right subtree.

2. Step 2: Confirm the visiting order

   The visiting order is Root, then Left subtree, then Right subtree.

3. Final Answer:

   Root, Left, Right -> Option D

4. Quick Check:

   Preorder = Root, Left, Right [OK]

Hint: Remember: Preorder starts with the root node first [OK]

Common Mistakes:

• Confusing preorder with inorder or postorder
• Visiting left subtree before root
• Visiting right subtree before left

2. Which of the following C# method signatures correctly represents a preorder traversal method for a binary tree node class named Node?

easy

A. void PreorderTraversal(Node root)

B. int PreorderTraversal(Node root)

C. Node PreorderTraversal()

D. void PreorderTraversal()

Solution

1. Step 1: Identify typical preorder traversal method signature

   Preorder traversal usually processes nodes and does not return a value, so it returns void and takes the root node as parameter.

2. Step 2: Match method signature options

   void PreorderTraversal(Node root) matches void return type and takes Node root parameter, which is correct.

3. Final Answer:

   void PreorderTraversal(Node root) -> Option A

4. Quick Check:

   Traversal method usually void with root parameter [OK]

Hint: Traversal methods usually return void and take root node [OK]

Common Mistakes:

• Using return type int instead of void
• Not passing root node as parameter
• Using method without parameters

3. Given the following binary tree:

    1
   / \
  2   3
 / \
4   5

What is the output of a preorder traversal printing node values?

medium

A. [1, 3, 2, 4, 5]

B. [4, 2, 5, 1, 3]

C. [1, 2, 4, 5, 3]

D. [2, 4, 5, 1, 3]

Solution

1. Step 1: Start preorder traversal at root node 1

   Visit root 1 first.

2. Step 2: Traverse left subtree of 1

   Visit node 2, then its left child 4, then right child 5.

3. Step 3: Traverse right subtree of 1

   Visit node 3.

4. Final Answer:

   [1, 2, 4, 5, 3] -> Option C

5. Quick Check:

   Preorder visits root, left subtree, right subtree [OK]

Hint: Visit root first, then left subtree, then right subtree [OK]

Common Mistakes:

• Mixing preorder with inorder or postorder output
• Visiting left subtree after right subtree
• Skipping leaf nodes

4. Consider this C# preorder traversal method:

void Preorder(Node root) {
  if (root == null) return;
  Preorder(root.Left);
  Console.Write(root.Value + " ");
  Preorder(root.Right);
}

What is the error in this code?

medium

A. It visits right subtree before left subtree

B. It visits left subtree before root, so not preorder

C. It does not check for null root

D. It prints node values incorrectly

Solution

1. Step 1: Analyze the visiting order in the method

   The method calls Preorder on left child first, then prints root value, then calls Preorder on right child.

2. Step 2: Compare with preorder definition

   Preorder requires visiting root first, then left, then right. This code visits left subtree before root, so it is actually inorder traversal.

3. Final Answer:

   It visits left subtree before root, so not preorder -> Option B

4. Quick Check:

   Preorder must print root before children [OK]

Hint: Print root value before recursive calls for preorder [OK]

Common Mistakes:

• Printing after left subtree call instead of before
• Confusing preorder with inorder traversal
• Missing null check (though present here)

5. You want to collect all node values of a binary tree in preorder traversal into a list in C#. Which code snippet correctly does this?

hard

A. void Preorder(Node root, List<int> result) {
if (root == null) return;
result.Add(root.Value);
Preorder(root.Left, result);
Preorder(root.Right, result);
}

B. void Preorder(Node root, List<int> result) {
if (root == null) return;
Preorder(root.Left, result);
result.Add(root.Value);
Preorder(root.Right, result);
}

C. void Preorder(Node root, List<int> result) {
if (root == null) return;
Preorder(root.Left, result);
Preorder(root.Right, result);
result.Add(root.Value);
}

D. void Preorder(Node root, List<int> result) {
if (root == null) return;
Preorder(root.Right, result);
result.Add(root.Value);
Preorder(root.Left, result);
}

Solution

1. Step 1: Understand preorder traversal collection

   Preorder means visit root first, so add root value to list before recursive calls.

2. Step 2: Check order of recursive calls

   After adding root, recursively traverse left subtree, then right subtree.

3. Final Answer:

   Code snippet that adds root value first, then left, then right -> Option A

4. Quick Check:

   Preorder = Add root, then left, then right [OK]

Hint: Add root value before recursive calls for preorder list [OK]

Common Mistakes:

• Adding node value after left subtree call (inorder)
• Adding node value after both subtree calls (postorder)
• Swapping left and right subtree calls