一天一大 leet(二叉树的序列化与反序列化)难度:困难 DAY-16

你可以将以下二叉树：
    1
   / 
  2   3
     / 
    4   5

序列化为 "[1,2,3,null,null,4,5]"

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */

/**
 * Encodes a tree to a single string.
 *
 * @param {TreeNode} root
 * @return {string}
 */
var serialize = function (root) {
  if (root === null || root.val === null) return 'null,'
  return root.val + ',' + serialize(root.left) + serialize(root.right)
}

/**
 * Decodes your encoded data to tree.
 *
 * @param {string} data
 * @return {TreeNode}
 */
var deserialize = function (data) {
  function tree(list) {
    if (list.length === 0);
    let val = list.shift()
    if (val === 'null') return null
    let node = new TreeNode(val)
    node.left = tree(list)
    node.right = tree(list)
    return node
  }
  let list = data.split(',')
  return tree(list)
}
/**
 * Your functions will be called as such:
 * deserialize(serialize(root));
 */

/**
 * Encodes a tree to a single string.
 *
 * @param {TreeNode} root
 * @return {string}
 */
var serialize = function (root) {
  const queue = [root]
  let res = []
  while (queue.length) {
    const node = queue.shift()
    if (node) {
      // 出列的节点 带出子节点入列
      res.push(node.val)
      queue.push(node.left) // 不管是不是null节点都入列
      queue.push(node.right)
    } else {
      res.push('X')
    }
  }
  return res.join(',')
}
/**
 * Decodes your encoded data to tree.
 *
 * @param {string} data
 * @return {TreeNode}
 */
var deserialize = function (data) {
  if (data == 'X') return null // 就一个'X'，只有一个null
  const list = data.split(',') // 序列化字符串转成list数组
  const root = new TreeNode(list[0]) //首项是根节点值，为它创建节点
  const queue = [root] // 初始放入root，待会出列考察
  let cursor = 1 // 从list第二项开始遍历
  while (cursor < list.length) {
    // 指针越界
    const node = queue.shift() // 父节点出列考察
    const leftVal = list[cursor] // 获取左子节点值
    const rightVal = list[cursor + 1] // 获取右子节点值
    if (leftVal !== 'X') {
      // 左子节点值是有效值
      const leftNode = new TreeNode(leftVal) // 创建节点
      node.left = leftNode // 成为当前出列节点的左子节点
      queue.push(leftNode) // 它是未来的爸爸，入列等待考察
    }
    if (rightVal !== 'X') {
      // 右子节点值是有效值
      const rightNode = new TreeNode(rightVal) // 创建节点
      node.right = rightNode // 成为当前出列节点的右子节点
      queue.push(rightNode) // 它是未来的爸爸，入列等待考察
    }
    cursor += 2 // 指针前进2位
  }
  return root // 返回根节点
}

// Definition for a binary tree node.
function TreeNode(val) {
  this.val = val
  this.left = this.right = null
}

// Your functions will be called as such:
deserialize(serialize(root))