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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the postorder traversal function recursively on the left child.

DSA C

void postorder(struct Node* root) {
    if (root == NULL) return;
    postorder(root->[1]);
    postorder(root->right);
    printf("%d ", root->data);
}

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

Aright

Bleft

Cdata

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using root->right instead of root->left for the first recursive call.

Trying to access root->data instead of a child pointer.

In postorder traversal, we first visit the left subtree, so we call postorder on root->left.

2fill in blank

medium

Complete the code to call the postorder traversal function recursively on the right child.

DSA C

void postorder(struct Node* root) {
    if (root == NULL) return;
    postorder(root->left);
    postorder(root->[1]);
    printf("%d ", root->data);
}

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

Aright

Bleft

Cdata

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using root->left instead of root->right for the second recursive call.

Trying to print root->right instead of calling postorder on it.

In postorder traversal, after visiting the left subtree, we visit the right subtree by calling postorder on root->right.

3fill in blank

hard

Fix the error in the print statement to correctly print the node's data.

DSA C

void postorder(struct Node* root) {
    if (root == NULL) return;
    postorder(root->left);
    postorder(root->right);
    printf("%[1]d ", root->data);
}

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

Af

Bc

Cs

Dd

Attempts:

3 left

💡 Hint

Common Mistakes

Using %f or %s which causes incorrect output or runtime errors.

Omitting the % sign or using wrong specifiers.

The correct format specifier for printing an integer in C is %d.

4fill in blank

hard

Fill both blanks to complete the postorder traversal function that prints nodes in left-right-root order.

DSA C

void postorder(struct Node* root) {
    if (root == [1]) return;
    postorder(root->[2]);
    postorder(root->right);
    printf("%d ", root->data);
}

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

ANULL

Broot

Cleft

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Checking root == root instead of root == NULL.

Calling postorder on root->right first instead of root->left.

The base case checks if root is NULL, and the first recursive call is on root->left.

5fill in blank

hard

Fill all three blanks to complete the postorder traversal function with correct base case, recursive calls, and print statement.

DSA C

void postorder(struct Node* root) {
    if (root == [1]) return;
    postorder(root->[2]);
    postorder(root->[3]);
    printf("%d ", root->data);
}

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

ANULL

Bleft

Cright

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up left and right child calls order.

Printing before recursive calls instead of after.

The base case is root == NULL, then recursive calls on root->left and root->right, then print root->data.

Practice

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

easy

A. Right subtree, Left subtree, Root node

B. Root node, Left subtree, Right subtree

C. Root node, Right subtree, Left subtree

D. Left subtree, Right 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: Apply the order to the traversal

   Following this order ensures all children are visited before the parent node.

3. Final Answer:

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

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 before children
• Swapping left and right subtree order

2. Which of the following C code snippets correctly implements the postorder traversal function for a binary tree node struct Node?

easy

A. void postorder(struct Node* root) { if(root) { printf("%d ", root->data); postorder(root->left); postorder(root->right); } }

B. void postorder(struct Node* root) { if(root) { postorder(root->left); postorder(root->right); printf("%d ", root->data); } }

C. void postorder(struct Node* root) { if(root) { postorder(root->right); postorder(root->left); printf("%d ", root->data); } }

D. void postorder(struct Node* root) { if(root) { postorder(root->left); printf("%d ", root->data); postorder(root->right); } }

Solution

1. Step 1: Check traversal order in code

   Postorder requires visiting left subtree, then right subtree, then root node.

2. Step 2: Match code to postorder steps

   void postorder(struct Node* root) { if(root) { postorder(root->left); postorder(root->right); printf("%d ", root->data); } } calls postorder on left, then right, then prints root data, matching postorder.

3. Final Answer:

   Correct postorder traversal code -> Option B

4. Quick Check:

   Left, Right, Root in code = void postorder(struct Node* root) { if(root) { postorder(root->left); postorder(root->right); printf("%d ", root->data); } } [OK]

Hint: Print root after left and right recursive calls [OK]

Common Mistakes:

• Printing root before children (preorder)
• Swapping left and right subtree calls
• Missing null check for root

3. Given the binary tree below:

    1
   / \
  2   3
 / \
4   5

What is the output of a postorder traversal?

medium

A. 2 4 5 3 1

B. 1 2 4 5 3

C. 4 5 2 3 1

D. 4 2 5 3 1

Solution

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

   Left subtree rooted at 2: postorder visits 4, then 5, then 2.

2. Step 2: Traverse right subtree and root

   Right subtree is 3 (leaf), then visit root 1 last.

3. Final Answer:

   4 5 2 3 1 -> Option C

4. Quick Check:

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

Hint: Visit children fully before printing parent [OK]

Common Mistakes:

• Printing root too early
• Mixing left and right subtree order
• Confusing preorder or inorder output

4. Consider the following C function for postorder traversal:

void postorder(struct Node* root) {
    if(root == NULL) return;
    postorder(root->right);
    postorder(root->left);
    printf("%d ", root->data);
}

What is the main issue with this code?

medium

A. It visits the right subtree before the left subtree, reversing the correct order

B. It does not check if root is NULL before recursion

C. It prints the root before visiting children

D. It causes infinite recursion

Solution

1. Step 1: Analyze the order of recursive calls

   The code calls postorder on right subtree first, then left subtree, which reverses the expected left-right order.

2. Step 2: Check printing position

   Printing root after children is correct, but subtree order is wrong.

3. Final Answer:

   Visits right subtree before left, reversing order -> Option A

4. Quick Check:

   Left before right needed, code does right before left [OK]

Hint: Left subtree call must come before right subtree call [OK]

Common Mistakes:

• Swapping left and right subtree calls
• Printing root too early
• Missing null check (not in this code though)

5. You have a binary tree where each node stores an integer. You want to collect all node values in postorder traversal into an array. Which approach correctly achieves this?

hard

A. Use recursion: traverse left subtree, traverse right subtree, then append root value to array

B. Use recursion: append root value, then traverse left and right subtrees

C. Use a queue to enqueue root first, then left and right children

D. Traverse the tree level by level and append nodes to array

Solution

1. Step 1: Understand postorder traversal for collection

   Postorder means visiting left subtree, right subtree, then root, so values must be appended in that order.

2. Step 2: Choose correct method to collect values

   Recursion that appends root after children ensures correct postorder sequence in the array.

3. Final Answer:

   Recursion: left, right, then append root -> Option A

4. Quick Check:

   Append after children = postorder collection [OK]

Hint: Append root after children for postorder array [OK]

Common Mistakes:

• Appending root before children (preorder)
• Using queue for level order, not postorder
• Confusing traversal types