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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to visit the left subtree first in postorder traversal.

DSA C#

void PostorderTraversal(Node node) {
    if (node == null) return;
    PostorderTraversal(node.[1]);
    // Continue traversal
}

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

Aroot

Bright

Cparent

Dleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'right' child first instead of 'left'.

In postorder traversal, we visit the left child first.

2fill in blank

medium

Complete the code to visit the right subtree second in postorder traversal.

DSA C#

void PostorderTraversal(Node node) {
    if (node == null) return;
    PostorderTraversal(node.left);
    PostorderTraversal(node.[1]);
    // Continue traversal
}

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

Aright

Bparent

Croot

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Visiting the root before the right subtree.

In postorder traversal, after the left subtree, we visit the right subtree.

3fill in blank

hard

Fix the error in the code to print the node value after visiting left and right subtrees.

DSA C#

void PostorderTraversal(Node node) {
    if (node == null) return;
    PostorderTraversal(node.left);
    PostorderTraversal(node.right);
    Console.[1](node.value);
}

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

AWriteLine

BPrint

CWrite

DDisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using Console.Write which does not add a newline.

Use Console.WriteLine to print the node value on its own line.

4fill in blank

hard

Fill both blanks to complete the recursive postorder traversal method.

DSA C#

void PostorderTraversal(Node node) {
    if (node == null) return;
    PostorderTraversal(node.[1]);
    PostorderTraversal(node.[2]);
    Console.WriteLine(node.value);
}

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

Aleft

Bright

Cparent

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping left and right children.

Postorder visits left subtree, then right subtree, then the node itself.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping node values to their postorder index.

DSA C#

Dictionary<int, int> postorderIndex = new Dictionary<int, int>();
int index = 0;

void PostorderTraversal(Node node) {
    if (node == null) return;
    PostorderTraversal(node.[1]);
    PostorderTraversal(node.[2]);
    postorderIndex[node.[3]] = index++;
}

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

Aleft

Bright

Cvalue

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using parent instead of children.

Using wrong property for dictionary key.

Traverse left and right children, then store the node's value with its postorder index.

Practice

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

easy

A. Left subtree, Right subtree, Root node

B. Root node, Left subtree, Right subtree

C. Root node, Right subtree, Left subtree

D. Right subtree, Left subtree, Root node

Solution

1. Step 1: Understand postorder traversal definition

   Postorder traversal visits the left subtree first, then the right subtree, and finally the root node.

2. Step 2: Confirm the visiting order

   This order ensures children nodes are processed before their parent node.

3. Final Answer:

   Left subtree, Right subtree, Root node -> Option A

4. Quick Check:

   Postorder = Left, Right, Root [OK]

Hint: Remember: postorder ends with the root node [OK]

Common Mistakes:

• Confusing postorder with preorder or inorder
• Visiting root node first instead of last
• Swapping left and right subtree order

2. Which of the following C# method signatures correctly represents a postorder traversal of a binary tree node named root?

easy

A. void PostorderTraversal(Node root) { PostorderTraversal(root.Right); PostorderTraversal(root.Left); Console.WriteLine(root.Value); }

B. void PostorderTraversal(Node root) { Console.WriteLine(root.Value); PostorderTraversal(root.Left); PostorderTraversal(root.Right); }

C. void PostorderTraversal(Node root) { PostorderTraversal(root.Left); PostorderTraversal(root.Right); Console.WriteLine(root.Value); }

D. void PostorderTraversal(Node root) { Console.WriteLine(root.Value); PostorderTraversal(root.Right); PostorderTraversal(root.Left); }

Solution

1. Step 1: Identify correct postorder call sequence

   Postorder traversal calls left subtree, then right subtree, then processes the root.

2. Step 2: Match method calls to postorder order

   void PostorderTraversal(Node root) { PostorderTraversal(root.Left); PostorderTraversal(root.Right); Console.WriteLine(root.Value); } calls left, then right, then prints root value, matching postorder traversal.

3. Final Answer:

   void PostorderTraversal(Node root) { PostorderTraversal(root.Left); PostorderTraversal(root.Right); Console.WriteLine(root.Value); } -> Option C

4. Quick Check:

   Left, Right, Root call order = void PostorderTraversal(Node root) { PostorderTraversal(root.Left); PostorderTraversal(root.Right); Console.WriteLine(root.Value); } [OK]

Hint: Postorder prints after left and right recursive calls [OK]

Common Mistakes:

• Printing root value before recursive calls
• Swapping left and right subtree calls
• Missing recursive calls for subtrees

3. Given the binary tree below, what is the output of a postorder traversal?

    1
   / \
  2   3
 / \
4   5

medium

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

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

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

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

Solution

1. Step 1: Traverse left subtree of root (1)

   Left subtree is rooted at 2, with children 4 and 5. Postorder visits 4, then 5, then 2.

2. Step 2: Traverse right subtree and root

   Right subtree is 3 (leaf), visited next. Finally, root 1 is visited last.

3. Final Answer:

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

4. Quick Check:

   Postorder = Left, Right, Root = [4,5,2,3,1] [OK]

Hint: Visit children fully before parent in postorder [OK]

Common Mistakes:

• Visiting root before children
• Swapping left and right subtree order
• Confusing preorder or inorder output

4. The following C# code attempts a postorder traversal but produces incorrect output. What is the error?

void Postorder(Node root) {
  if (root == null) return;
  Console.WriteLine(root.Value);
  Postorder(root.Left);
  Postorder(root.Right);
}

medium

A. The root value is printed before traversing subtrees

B. The base case for null root is missing

C. Left and right subtree calls are swapped

D. The method does not return any value

Solution

1. Step 1: Analyze the order of operations in code

   The code prints root value first, then traverses left and right subtrees.

2. Step 2: Compare with postorder traversal rules

   Postorder requires visiting left and right subtrees before printing root value.

3. Final Answer:

   Root value printed before subtrees traversal -> Option A

4. Quick Check:

   Postorder prints after children, not before [OK]

Hint: Print root after recursive calls for postorder [OK]

Common Mistakes:

• Printing root before recursive calls
• Ignoring null base case (though present here)
• Swapping left and right subtree calls

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

hard

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

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

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

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

Solution

1. Step 1: Confirm traversal order in code

   void Postorder(Node root, List<int> result) { if(root == null) return; Postorder(root.Left, result); Postorder(root.Right, result); result.Add(root.Value); } calls left subtree, then right subtree, then adds root value to list, matching postorder.

2. Step 2: Verify list collection logic

   Adding root value after recursive calls ensures correct postorder sequence in the list.

3. Final Answer:

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

4. Quick Check:

   Left, Right, Root with Add after recursion [OK]

Hint: Add node value after left and right recursive calls [OK]

Common Mistakes:

• Adding root value before recursive calls
• Swapping left and right subtree calls
• Not handling null root properly