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Tree BFS

Guide 2 of 6 · Path 2 of 6

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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 Zigzag Level Order Traversal

Problem (restated)

Level-order values, but alternate left→right and right→left each level.

Intuition

BFS levels; reverse odd levels (or use deque direction).

Approaches

Level BFS + reverse

Tested only
Time O(n)Space O(w)

Idea. collect level as usual; if depth odd reverse before push.

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

Trade-offs. Deque insert-front is optional sugar; reverse is clear.

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 zigzagLevelOrder(root: TreeNode | null): number[][] {
  if (!root) return [];
  const res: number[][] = [];
  const q: TreeNode[] = [root];
  let leftToRight = true;
  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);
    }
    if (!leftToRight) level.reverse();
    res.push(level);
    leftToRight = !leftToRight;
  }
  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 zigzagLevelOrder(root: TreeNode | null): number[][] {
  if (!root) return [];
  const res: number[][] = [];
  const q: TreeNode[] = [root];
  let leftToRight = true;
  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);
    }
    if (!leftToRight) level.reverse();
    res.push(level);
    leftToRight = !leftToRight;
  }
  return res;
}

Template connection

Tree BFS with level post-process.

Reflection