Zigzag Level Order Traversal in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue with the root node for level order traversal.

DSA Python

queue = [[1]]  # Start with root node in queue

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

Aroot

B[]

CNone

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Starting with an empty list instead of root

Using None instead of root

We start the traversal by putting the root node inside the queue.

2fill in blank

medium

Complete the code to toggle the direction for zigzag traversal after each level.

DSA Python

left_to_right = not [1]

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

Aleft_to_right

Bqueue

Clevel

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Toggling wrong variable

Not toggling direction at all

We flip the boolean variable left_to_right to change direction after each level.

3fill in blank

hard

Fix the error in the code to append children nodes correctly during traversal.

DSA Python

if node.left:
    [1].append(node.left)
if node.right:
    queue.append(node.right)

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

Astack

Bqueue

Clevel_nodes

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Appending children to wrong list

Using undefined variable

Children nodes must be added to the queue to be processed in next levels.

4fill in blank

hard

Fill both blanks to collect node values in correct order for zigzag traversal.

DSA Python

if left_to_right:
    level_values.[1](node.val)
else:
    level_values.[2](0, node.val)

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

Aappend

Binsert

Cpop

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop or remove instead of append/insert

Appending in both directions

When going left to right, append values at the end. When right to left, insert at start to reverse order.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps node values to their depths.

DSA Python

depth_map = { [1]: [2] for [3] in nodes_with_depth }

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

Anode.val

Bdepth

Cnode, depth

Dnodes_with_depth

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names

Swapping keys and values

This comprehension creates a dictionary where keys are node values and values are their depths.

Practice

1. What does the Zigzag Level Order Traversal of a binary tree do differently compared to a normal level order traversal?

easy

A. It traverses the tree using depth-first search.

B. It visits nodes only at even levels.

C. It visits nodes in reverse order of their values.

D. It alternates the direction of traversal at each level, left to right then right to left.

Solution

1. Step 1: Understand normal level order traversal

   Normal level order traversal visits nodes level by level from left to right.

2. Step 2: Identify zigzag difference

   Zigzag traversal changes direction each level: first left to right, then right to left, and so on.

3. Final Answer:

   It alternates the direction of traversal at each level, left to right then right to left. -> Option D

4. Quick Check:

   Zigzag = alternate directions [OK]

Hint: Remember zigzag means direction flips each level [OK]

Common Mistakes:

• Thinking it only visits even levels
• Confusing with reverse value order
• Mixing with depth-first search

2. Which of the following Python code snippets correctly initializes a queue for level order traversal in zigzag pattern?

easy

A. from collections import deque
queue = deque([root])

B. queue = list(root)

C. queue = set(root)

D. queue = root.pop()

Solution

1. Step 1: Identify correct queue initialization

   For level order traversal, a queue is needed; deque from collections is ideal.

2. Step 2: Check each option

   from collections import deque queue = deque([root]) uses deque with root node inside, correct for BFS. Others misuse list, set, or pop method.

3. Final Answer:

   from collections import deque\nqueue = deque([root]) -> Option A

4. Quick Check:

   Use deque for queue in BFS [OK]

Hint: Use deque with root inside for BFS queue [OK]

Common Mistakes:

• Using list instead of deque for queue
• Using set which is unordered
• Calling pop on root which is a node, not a list

3. What is the output of the zigzag level order traversal for the binary tree below?

    1
   / \
  2   3
 / \   \
4   5   6

Assuming the traversal starts left to right at level 0.

medium

A. [[1], [2, 3], [6, 5, 4]]

B. [[1], [3, 2], [6, 5, 4]]

C. [[1], [3, 2], [4, 5, 6]]

D. [[1], [2, 3], [4, 5, 6]]

Solution

1. Step 1: Traverse level 0 left to right

   Level 0 has node 1, so output is [1].

2. Step 2: Traverse level 1 right to left

   Level 1 nodes are 2 and 3; reversed order is [3, 2].

3. Step 3: Traverse level 2 left to right

   Level 2 nodes are 4, 5, 6; order is [4, 5, 6].

4. Final Answer:

   [[1], [3, 2], [4, 5, 6]] -> Option C

5. Quick Check:

   Zigzag levels alternate direction [OK]

Hint: Alternate direction each level starting left to right [OK]

Common Mistakes:

• Not reversing the order on odd levels
• Reversing the wrong levels
• Mixing node values order

4. The following code attempts zigzag level order traversal but produces incorrect output. What is the main error?

def zigzag(root):
    if not root:
        return []
    from collections import deque
    queue = deque([root])
    result = []
    left_to_right = True
    while queue:
        level_size = len(queue)
        level_nodes = []
        for _ in range(level_size):
            node = queue.popleft()
            if left_to_right:
                level_nodes.append(node.val)
            else:
                level_nodes.insert(0, node.val)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
        result.append(level_nodes)
        # Missing toggle of direction here
    return result

medium

A. The direction flag 'left_to_right' is never toggled inside the loop.

B. The queue is not initialized correctly.

C. Nodes are appended to the wrong end of the queue.

D. The function does not check if root is null.

Solution

1. Step 1: Analyze direction toggle usage

   The variable 'left_to_right' controls order but is never changed inside the loop.

2. Step 2: Understand effect of missing toggle

   Without toggling, traversal direction stays same every level, so no zigzag effect.

3. Final Answer:

   The direction flag 'left_to_right' is never toggled inside the loop. -> Option A

4. Quick Check:

   Toggle direction each level [OK]

Hint: Toggle direction flag after each level [OK]

Common Mistakes:

• Forgetting to toggle direction
• Incorrect queue initialization
• Misusing queue append/popleft

5. Given a binary tree, how would you modify the zigzag level order traversal to return the sum of node values at each level instead of the nodes themselves?

hard

A. Reverse the entire result list at the end.

B. Accumulate the sum of values in 'level_nodes' before appending to result.

C. Use a stack instead of a queue to store nodes.

D. Append node values directly to result without using 'level_nodes'.

Solution

1. Step 1: Understand current storage of nodes per level

   Currently, nodes are collected in 'level_nodes' list for each level.

2. Step 2: Modify to sum values

   Instead of appending the list, calculate sum(level_nodes) and append that sum to result.

3. Final Answer:

   Accumulate the sum of values in 'level_nodes' before appending to result. -> Option B

4. Quick Check:

   Sum level nodes before appending [OK]

Hint: Sum values in level list before adding to result [OK]

Common Mistakes:

• Appending nodes instead of sums
• Using stack instead of queue incorrectly
• Reversing result instead of levels