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64. Binary Tree Level Order Traversal

mediumAsked at Reddit

Return level-order traversal of a binary tree as a list of lists. Reddit uses this BFS warm-up to test queue technique — the same shape used in their comment-tree renderer which paginates by depth level.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit comments-team on-site question.
  • Blind (2025-11)— Reported on Reddit comments-platform rounds.

Problem

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

Constraints

  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000

Examples

Example 1

Input
root = [3,9,20,null,null,15,7]
Output
[[3],[9,20],[15,7]]

Example 2

Input
root = [1]
Output
[[1]]

Example 3

Input
root = []
Output
[]

Approaches

1. BFS with level marker

Queue + null markers between levels.

Time
O(n)
Space
O(w)
// Anti-pattern: null markers are bug-prone.

Tradeoff: Works but error-prone.

2. BFS with size-per-level (optimal)

Per iteration, capture queue size, drain that many. Push children. Each drain produces one level.

Time
O(n)
Space
O(w)
function levelOrder(root) {
  if (!root) return [];
  const out = [];
  const q = [root];
  while (q.length) {
    const size = q.length;
    const level = [];
    for (let i = 0; i < size; i++) {
      const n = q.shift();
      level.push(n.val);
      if (n.left) q.push(n.left);
      if (n.right) q.push(n.right);
    }
    out.push(level);
  }
  return out;
}

Tradeoff: Linear. The 'capture size at start of each level' pattern is the canonical BFS-level trick.

Reddit-specific tips

Reddit interviewers want the size-per-level trick. Bonus signal: connect to their comment-tree pagination — render level 0 immediately, paginate deeper levels via 'Show more comments' buttons.

Common mistakes

  • Reading q.length inside the inner loop (it changes as you push children).
  • Pushing null children (always check).
  • Returning a flat array instead of a list-of-lists.

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • Zigzag (LC 103) — reverse alternate levels.
  • Bottom-up (LC 107) — same algorithm, reverse output.
  • Right side view (LC 199) — last node per level.

Solve it now

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Output

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FAQ

Why capture size before draining?

We need to know exactly how many nodes are on this level before adding their children to the queue.

Could DFS do this?

Yes — DFS with depth parameter, appending to out[depth]. Same complexity, different traversal order.

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