Demo preview: These solutions pass the automated test suite but have not been human-reviewed. Treat trade-offs and prose as draft.

Mediumtree-bfs

Binary Tree Level Order Traversal

Problem (restated)

Return the level order traversal of a binary tree values (left to right, level by level).

Intuition

Queue; process exact level size each round.

Approaches

Level BFS

Tested only

Time O(n)Space O(w)

Idea. BFS with size snapshot per level.

Walkthrough. [3,9,20,null,null,15,7] → [[3],[9,20],[15,7]].

Trade-offs. BFS natural; DFS with depth lists also works.

Solution

export class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val = 0, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val; this.left = left; this.right = right;
  }
}

export function levelOrder(root: TreeNode | null): number[][] {
  if (!root) return [];
  const res: number[][] = [];
  const q: TreeNode[] = [root];
  while (q.length) {
    const size = q.length;
    const level: number[] = [];
    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);
    }
    res.push(level);
  }
  return res;
}

export class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val = 0, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val; this.left = left; this.right = right;
  }
}

export function levelOrder(root: TreeNode | null): number[][] {
  if (!root) return [];
  const res: number[][] = [];
  const q: TreeNode[] = [root];
  while (q.length) {
    const size = q.length;
    const level: number[] = [];
    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);
    }
    res.push(level);
  }
  return res;
}

Template connection

Tree BFS levels.

Reflection

Which pattern gave this away within 90 seconds?
What changed from the standard template?
What would break the current solution?