Tree Traversal Preorder Root Left Right in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the root node's value first in preorder traversal.

DSA C#

Console.WriteLine(node[1]);

Drag options to blanks, or click blank then click option'

A.Left

B.Value

C.Right

D.Parent

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to print node.Left or node.Right instead of node.Value.

Using node.Parent which is not part of preorder traversal.

In preorder traversal, we first visit the root node's value, which is accessed by the .Value property.

2fill in blank

medium

Complete the code to recursively traverse the left subtree in preorder.

DSA C#

if (node.Left != null) [1](node.Left);

Drag options to blanks, or click blank then click option'

ATraverseInorder

BPrintNode

CTraversePostorder

DTraversePreorder

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the wrong traversal function like TraverseInorder or TraversePostorder.

Not calling the function recursively.

In preorder traversal, we recursively call the same preorder function on the left child.

3fill in blank

hard

Fix the error in the preorder traversal to correctly visit the right subtree after the left.

DSA C#

if (node.Right != null) [1](node.Right);

Drag options to blanks, or click blank then click option'

ATraversePreorder

BPrintNode

CTraversePostorder

DTraverseInorder

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a different traversal function on the right subtree.

Forgetting to traverse the right subtree.

After visiting the root and left subtree, preorder traversal visits the right subtree by calling TraversePreorder on node.Right.

4fill in blank

hard

Fill both blanks to complete the preorder traversal method visiting root, left, then right.

DSA C#

public void TraversePreorder(Node node) {
    if (node == null) return;
    Console.WriteLine(node[1]);
    if (node.Left != null) [2](node.Left);
    if (node.Right != null) TraversePreorder(node.Right);
}

Drag options to blanks, or click blank then click option'

A.Value

BTraverseInorder

CTraversePreorder

D.Left

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property like .Left instead of .Value for printing.

Calling the wrong traversal function on left child.

We print the node's value first, then recursively call TraversePreorder on the left child.

5fill in blank

hard

Fill all three blanks to implement preorder traversal printing root, then left subtree, then right subtree.

DSA C#

public void TraversePreorder(Node node) {
    if (node == null) return;
    Console.WriteLine(node[1]);
    if (node.Left != null) [2](node.Left);
    if (node.Right != null) [3](node.Right);
}

Drag options to blanks, or click blank then click option'

A.Value

BTraversePreorder

CTraverseInorder

DPrintNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using TraverseInorder or PrintNode instead of TraversePreorder for recursion.

Printing node.Left or node.Right instead of node.Value.

We print node.Value, then recursively call TraversePreorder on left and right children.

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