Zigzag Level Order Traversal in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue with the root node.

DSA Go

queue := []*TreeNode[1]

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

Aroot

Bnil

C[]

DTreeNode{}

Attempts:

3 left

💡 Hint

Common Mistakes

Starting with an empty queue

Using nil instead of root

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

2fill in blank

medium

Complete the code to toggle the direction for zigzag traversal.

DSA Go

leftToRight = ![1]

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

Adirection

BrightToLeft

CleftToRight

Dflag

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable name

Not toggling the direction

We flip the boolean leftToRight to change the traversal direction after each level.

3fill in blank

hard

Fix the error in the code to append child nodes to the queue correctly.

DSA Go

if node.Left != nil {
    queue = append(queue, [1])
}
if node.Right != nil {
    queue = append(queue, node.Right)
}

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

Anode.Left

Bnode.Right

Cnil

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Appending wrong child

Appending nil

We must append the left child node to the queue if it exists.

4fill in blank

hard

Fill both blanks to correctly reverse the level slice when needed.

DSA Go

if !leftToRight {
    for i, j := 0, len(level)-1; i < j; i, j = i+1, [1] {
        level[i], level[j] = level[j], level[i]
    }
}

queue = queue[[2]:]

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

Aj-1

Blen(level)

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrect loop decrement

Wrong queue slicing

We decrement j by 1 in the loop and remove the first element from the queue after processing.

5fill in blank

hard

Fill all three blanks to build the zigzag level order traversal correctly.

DSA Go

result := [][]int{}
queue := []*TreeNode[1]
leftToRight := true

for len(queue) > 0 {
    levelSize := len(queue)
    level := make([]int, levelSize)
    for i := 0; i < levelSize; i++ {
        node := queue[0]
        queue = queue[[2]:]
        if leftToRight {
            level[i] = node.Val
        } else {
            level[levelSize-1-[3]] = node.Val
        }
        if node.Left != nil {
            queue = append(queue, node.Left)
        }
        if node.Right != nil {
            queue = append(queue, node.Right)
        }
    }
    result = append(result, level)
    leftToRight = !leftToRight
}

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

Aroot

B1

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Wrong queue initialization

Incorrect index usage

Not removing nodes from queue

Start queue with root, remove one node each iteration, and place values correctly based on direction.

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 alternates the direction of traversal at each level, switching between left-to-right and right-to-left.

B. It only traverses the left subtree of the tree.

C. It visits nodes in a random order at each level.

D. It traverses nodes only in right-to-left order for all levels.

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 every level: first left-to-right, then right-to-left, and so on.

3. Final Answer:

   It alternates the direction of traversal at each level, switching between left-to-right and right-to-left. -> Option A

4. Quick Check:

   Zigzag = alternate directions [OK]

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

Common Mistakes:

• Thinking traversal is always left-to-right
• Confusing zigzag with random order
• Ignoring direction change at each level

2. Which of the following Go code snippets correctly initializes a queue for level order traversal in Zigzag Level Order Traversal?

easy

A. queue := make([]int, 0)

B. queue := []int{root}

C. queue := map[int]*TreeNode{}

D. queue := []*TreeNode{root}

Solution

1. Step 1: Identify the queue type needed

   Queue should hold pointers to TreeNode structs, so []*TreeNode is correct.

2. Step 2: Check each option's correctness

   queue := []*TreeNode{root} uses []*TreeNode with root node, correct for traversal. Options A and B use int slices, wrong type. queue := map[int]*TreeNode{} uses map, not queue.

3. Final Answer:

   queue := []*TreeNode{root} -> Option D

4. Quick Check:

   Queue holds TreeNode pointers = queue := []*TreeNode{root} [OK]

Hint: Queue must hold TreeNode pointers, not ints or maps [OK]

Common Mistakes:

• Using int slices instead of TreeNode pointers
• Using map instead of slice for queue
• Not initializing queue with root node

3. Given the binary tree:

    1
   / \
  2   3
 / \   \
4   5   6

What is the output of Zigzag Level Order Traversal?

medium

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

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

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

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

Solution

1. Step 1: Traverse level 0 and 1 with direction

   Level 0: left-to-right -> [1]. Level 1: right-to-left -> [3, 2].

2. Step 2: Traverse level 2 left-to-right

   Level 2 nodes: 4, 5, 6 from left to right -> [4, 5, 6].

3. Final Answer:

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

4. Quick Check:

   Zigzag levels alternate directions = [[1], [3, 2], [4, 5, 6]] [OK]

Hint: Alternate direction each level, left-to-right then right-to-left [OK]

Common Mistakes:

• Not reversing nodes on odd levels
• Reversing nodes on even levels incorrectly
• Mixing node order at last level

4. Consider this Go code snippet for Zigzag Level Order Traversal:

for len(queue) > 0 {
    levelSize := len(queue)
    var level []int
    for i := 0; i < levelSize; i++ {
        node := queue[0]
        queue = queue[1:]
        level = append(level, node.Val)
        if node.Left != nil {
            queue = append(queue, node.Left)
        }
        if node.Right != nil {
            queue = append(queue, node.Right)
        }
    }
    if leftToRight {
        result = append(result, level)
    } else {
        reverse(level)
        result = append(result, level)
    }
    leftToRight = !leftToRight
}

What is the main bug in this code?

medium

A. The reverse function is called but not defined or implemented.

B. The queue is not updated correctly after popping the first element.

C. The variable leftToRight is never initialized before use.

D. The level slice is appended before processing child nodes.

Solution

1. Step 1: Check queue update correctness

   Queue is updated by slicing off first element correctly, no bug here.

2. Step 2: Verify reverse function usage

   reverse(level) is called but no definition or implementation is shown, causing compile error.

3. Final Answer:

   The reverse function is called but not defined or implemented. -> Option A

4. Quick Check:

   Undefined reverse function causes error [OK]

Hint: Ensure all helper functions like reverse are implemented [OK]

Common Mistakes:

• Assuming reverse is built-in in Go
• Ignoring initialization of leftToRight (usually done before loop)
• Misunderstanding queue slicing syntax

5. You want to implement Zigzag Level Order Traversal but optimize space by avoiding reversing slices. Which approach is best?

hard

A. Use a queue and reverse the slice after each level as usual.

B. Use two stacks to alternate pushing nodes in correct order per level.

C. Use recursion with a global list and reverse at the end.

D. Traverse all nodes left-to-right and then reverse the entire result.

Solution

1. Step 1: Understand reversing cost

   Reversing slices after each level adds extra time and space overhead.

2. Step 2: Use two stacks for order control

   Two stacks let you push child nodes in order that naturally produces zigzag traversal without reversing.

3. Final Answer:

   Use two stacks to alternate pushing nodes in correct order per level. -> Option B

4. Quick Check:

   Two stacks avoid reversing slices [OK]

Hint: Two stacks method avoids reversing slices [OK]

Common Mistakes:

• Relying on reversing slices every level
• Using recursion without order control
• Reversing entire result instead of per level