TreeNode.js

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

  static parse(arr) {
    if (arr.length === 0) return null;
    const root = new TreeNode(arr[0]);
    const queue = [root];
    for (let i = 1; i < arr.length; i += 2) {
      const node = queue.shift();
      if (arr[i] !== null) {
        node.left = new TreeNode(arr[i]);
        queue.push(node.left);
      }
      if (arr[i + 1] !== null) {
        node.right = new TreeNode(arr[i + 1]);
        queue.push(node.right);
      }
    }
    return root;
  }

  static toArray(treeNode) {
    const result = [];
    if (!treeNode) {
      return result;
    }

    const queue = [treeNode];

    while (queue.length > 0) {
      const node = queue.shift();
      if (node) {
        result.push(node.val);
        queue.push(node.left);
        queue.push(node.right);
      } else {
        result.push(null);
      }
    }

    while (result.length > 0 && result[result.length - 1] === null) {
      result.pop();
    }

    return result;
  }
}