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Binary Tree Level Order Traversal – Solution & Complexity

Interview solutions
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Binary Tree Level Order Traversal

Solution Walkthrough

  1. Step 1: Recognize the Pattern

    The key pattern is: BFS processes one queue level at a time. Identify the state and invariant before coding.

  2. Step 2: Build the Algorithm

    Advance one state transition at a time. Mark or update state before exploring dependent work.

  3. Step 3: Check Edge Cases

    Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.

  4. Step 4: Solution and Complexity

    Time: O(n). Space: O(n).

    Java/Go/Rust below are reference solutions for learning the pattern in those languages; only Python and JavaScript can be run and submitted for this problem, since the remote judge doesn't support the TreeNode structure yet.

    def level_order(root: TreeNode) -> list[list[int]]:
    if root is None:
        return []
    answer, queue, head = [], [root], 0
    while head < len(queue):
        level = []
        for _ in range(len(queue) - head):
            node = queue[head]; head += 1
            level.append(node.val)
            if node.left: queue.append(node.left)
            if node.right: queue.append(node.right)
        answer.append(level)
    return answer
Learn the Tree Traversal pattern: recognition cues, invariant, and ordered practice →

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See also

Practice Binary Tree Level Order Traversal online (run code in the browser)Binary Tree Right Side View solution (medium)Binary Tree Zigzag Level Order Traversal solution (medium)01 Matrix solution (medium)Balanced Binary Tree solution (easy)All coding interview problems
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