How do you solve Binary Tree Zigzag Level Order Traversal on LeetCode?

Binary Tree Zigzag Level Order Traversal (LeetCode #103) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Using a deque allows for efficient insertion at both ends, which is crucial for zigzag traversal.

What is the time complexity of Binary Tree Zigzag Level Order Traversal?

The optimal solution for Binary Tree Zigzag Level Order Traversal runs in O(n) time and uses O(n) space. The time complexity is O(n) because we visit each node exactly once. The space complexity is O(n) due to the storage of nodes in the queue.

What is the brute force solution for Binary Tree Zigzag Level Order Traversal?

The brute force approach for Binary Tree Zigzag Level Order Traversal is Brute Force, with O(n²) time complexity and O(1) space complexity. We can traverse the tree level by level, storing values in a list. For each level, we can reverse the order of values based on whether the level is odd or even. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Binary Tree Zigzag Level Order Traversal?

Binary Tree Zigzag Level Order Traversal uses the following concepts: Tree, Breadth-First Search, Binary Tree. The recommended patterns to study are: Breadth-First Search (BFS), Deque for efficient insertion/removal.

What are the interview tips for Binary Tree Zigzag Level Order Traversal?

Always clarify the problem requirements and edge cases before diving into coding. Think about the data structures that can help you solve the problem efficiently. Practice explaining your thought process as you code — it helps in interviews.

What are common mistakes when solving Binary Tree Zigzag Level Order Traversal?

Students often forget to handle the case of an empty tree, leading to null pointer exceptions. Not using a deque or list correctly can lead to unnecessary reversals, impacting performance.

#103

Binary Tree Zigzag Level Order Traversal

Medium

TreeBreadth-First SearchBinary TreeBreadth-First Search (BFS)Deque for efficient insertion/removal

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

We can use a queue for level order traversal and a flag to alternate the order of insertion. This way, we avoid reversing the list at each level, making it more efficient.

⚙️

Algorithm

3 steps

1Step 1: Initialize a queue and add the root node.
2Step 2: Use a boolean flag to track the direction of insertion.
3Step 3: For each level, collect values in the appropriate order based on the flag.

solution.py25 lines

1# Full working Python code
2from collections import deque
3
4def zigzagLevelOrder(root):
5    if not root:
6        return []
7    result = []
8    queue = deque([root])
9    left_to_right = True
10    while queue:
11        level_size = len(queue)
12        current_level = deque()
13        for _ in range(level_size):
14            node = queue.popleft()
15            if left_to_right:
16                current_level.append(node.val)
17            else:
18                current_level.appendleft(node.val)
19            if node.left:
20                queue.append(node.left)
21            if node.right:
22                queue.append(node.right)
23        result.append(list(current_level))
24        left_to_right = not left_to_right
25    return result

ℹ

Complexity note: The time complexity is O(n) because we visit each node exactly once. The space complexity is O(n) due to the storage of nodes in the queue.

1Using a deque allows for efficient insertion at both ends, which is crucial for zigzag traversal.
2Maintaining a boolean flag to track the direction of traversal simplifies the logic.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.