Boundary Traversal of Binary Tree in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add the root node's data to the boundary list.

DSA C#

boundary.Add([1].data);

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

Aroot

Bnode

Ctree

DrootNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name other than 'root' which is not defined.

Trying to add the whole node instead of its data.

The root node is the starting point of the boundary traversal, so we add root.data to the list.

2fill in blank

medium

Complete the code to check if a node is a leaf node.

DSA C#

bool isLeaf = (node.left == null && node.[1] == null);

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Achild

Bright

Cnext

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'child' or 'next' which are not valid properties.

Checking only one child instead of both.

A leaf node has no left or right child, so both must be null.

3fill in blank

hard

Fix the error in the code to traverse the left boundary excluding leaf nodes.

DSA C#

while (curr != null && !isLeaf(curr)) {
    boundary.Add(curr.data);
    curr = curr.[1] != null ? curr.left : curr.right;
}

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Aleft

Bright

Cparent

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Using curr.right to move left boundary which is incorrect.

Using invalid property names like 'parent' or 'child'.

To traverse the left boundary, move to the left child if it exists; otherwise, move to the right child.

4fill in blank

hard

Fill both blanks to traverse the right boundary excluding leaf nodes in reverse order.

DSA C#

Stack<int> stack = new Stack<int>();
while (curr != null && !isLeaf(curr)) {
    stack.Push(curr.data);
    curr = curr.[1] != null ? curr.[1] : curr.[2];
}
while (stack.Count > 0) {
    boundary.Add(stack.Pop());
}

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Aright

Bleft

Cparent

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping left and right child in traversal.

Not using a stack to reverse the right boundary nodes.

For right boundary traversal, move to right child if it exists; otherwise, move to left child.

5fill in blank

hard

Fill all three blanks to add all leaf nodes from left to right.

DSA C#

void addLeaves(Node node, List<int> boundary) {
    if (node == null) return;
    if (isLeaf(node)) {
        boundary.Add(node.[1]);
        return;
    }
    addLeaves(node.[2], boundary);
    addLeaves(node.[3], boundary);
}

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

Adata

Bleft

Cright

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'data' for node's value.

Swapping left and right child traversal order.

Add the node's data if it's a leaf, then recursively add leaves from left and right children.

Practice

1. What does the Boundary Traversal of a binary tree include?

easy

A. Only the root node

B. Left boundary, leaves, and right boundary nodes in order

C. All nodes in level order

D. Only the leaf nodes of the tree

Solution

1. Step 1: Understand boundary traversal components

   Boundary traversal includes nodes on the left edge, all leaf nodes, and nodes on the right edge.

2. Step 2: Combine these parts in order

   Traverse left boundary top-down, then leaves left to right, then right boundary bottom-up.

3. Final Answer:

   Left boundary, leaves, and right boundary nodes in order -> Option B

4. Quick Check:

   Boundary traversal = left + leaves + right [OK]

Hint: Remember: left edge, leaves, right edge in sequence [OK]

Common Mistakes:

• Including root multiple times
• Missing leaf nodes
• Reversing right boundary order

2. Which of the following C# method signatures is correct for a function that performs boundary traversal on a binary tree node root and returns a list of integers?

easy

A. public void BoundaryTraversal(int root)

B. public int BoundaryTraversal(Node root)

C. public List<int> BoundaryTraversal(Node root)

D. public List BoundaryTraversal(Node root)

Solution

1. Step 1: Check return type and parameter

   Boundary traversal returns a list of node values, so return type should be List<int> and parameter should be Node.

2. Step 2: Validate method signature syntax

   public List<int> BoundaryTraversal(Node root) matches correct syntax: public List<int> BoundaryTraversal(Node root)

3. Final Answer:

   public List<int> BoundaryTraversal(Node root) -> Option C

4. Quick Check:

   Return list<int> and input Node root [OK]

Hint: Return list of ints, input is Node root [OK]

Common Mistakes:

• Using void return type
• Wrong parameter type (int instead of Node)
• Missing generic type in List

3. Given the following binary tree:

       1
      / \
     2   3
    / \   \
   4   5   6
      / \  /
     7  8 9

What is the boundary traversal output?

medium

A. [1, 2, 4, 7, 8, 9, 3, 6]

B. [1, 2, 4, 7, 8, 9, 6]

C. [1, 2, 4, 7, 8, 9, 6, 3]

D. [1, 2, 4, 5, 7, 8, 9, 6, 3]

Solution

1. Step 1: Identify left boundary nodes

   Left boundary (excluding leaves): 1, 2

2. Step 2: Identify leaf nodes left to right

   Leaves: 4, 7, 8, 9

3. Step 3: Identify right boundary nodes bottom-up

   Right boundary (excluding leaves): 3, 6

4. Step 4: Combine nodes without duplicates

   Boundary traversal: 1, 2, 4, 7, 8, 9, 3, 6

5. Final Answer:

   [1, 2, 4, 7, 8, 9, 3, 6] -> Option A

6. Quick Check:

   Left + leaves + right = [1,2,4,7,8,9,3,6] [OK]

Hint: Left edge + leaves + right edge reversed [OK]

Common Mistakes:

• Including leaf nodes twice
• Missing right boundary node 3
• Wrong order of leaves

4. Consider this C# code snippet for boundary traversal:

void PrintLeftBoundary(Node node) {
  if (node == null) return;
  Console.Write(node.Data + " ");
  if (node.Left != null) {
    PrintLeftBoundary(node.Left);
  } else if (node.Right != null) {
    PrintLeftBoundary(node.Right);
  }
}

What is the bug in this code?

medium

A. It skips left boundary nodes

B. It does not print the root node

C. It causes infinite recursion on leaf nodes

D. It prints leaf nodes which should be excluded

Solution

1. Step 1: Analyze leaf node printing

   The code prints node data before recursive call even if node is leaf, so leaf nodes get printed here.

2. Step 2: Understand boundary traversal rules

   Left boundary function should exclude leaf nodes to avoid duplicates in leaf printing step.

3. Final Answer:

   It prints leaf nodes which should be excluded -> Option D

4. Quick Check:

   Left boundary excludes leaves [OK]

Hint: Exclude leaves in left boundary print [OK]

Common Mistakes:

• Printing leaves in left boundary
• Not checking for null nodes
• Confusing left and right child checks

5. You want to perform boundary traversal on a binary tree but avoid printing duplicate nodes when the tree has only one node or skewed structure. Which approach is best?

hard

A. Print root, then left boundary excluding leaves, then leaves excluding root, then right boundary excluding leaves and root

B. Print all nodes in preorder traversal

C. Print leaves only once and ignore boundaries

D. Print left boundary including leaves, then right boundary including leaves

Solution

1. Step 1: Avoid duplicates by excluding leaves in boundaries

   Exclude leaves from left and right boundary to prevent duplicates with leaf traversal.

2. Step 2: Handle root separately

   Print root once at start, exclude it from right boundary to avoid repetition.

3. Step 3: Combine all parts carefully

   Print root, left boundary (no leaves), leaves (excluding root), right boundary (no leaves or root) for correct order and no duplicates.

4. Final Answer:

   Print root, then left boundary excluding leaves, then leaves excluding root, then right boundary excluding leaves and root -> Option A

5. Quick Check:

   Exclude duplicates by careful boundary and leaf separation [OK]

Hint: Exclude leaves in boundaries, print root once [OK]

Common Mistakes:

• Printing root multiple times
• Including leaves in boundaries causing duplicates
• Ignoring skewed tree edge cases