/**
 * 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}
 */

const serialize = function(root) {
  const str = [];
  tostr(root, str);
  return str;
};
const tostr = function(root, str) {
  if (root === null) {
    str.push('X');
  } else {
    str.push(root.val);
    tostr(root.left, str);
    tostr(root.right, str);
  }
}

/**
 * Decodes your encoded data to tree.
 *
 * @param {string} data
 * @return {TreeNode}
 */
const deserialize = function(data) {
  var val = data.shift();
  if (val === "X") {
    return null;
  } else {
    const node = new TreeNode(val);
    node.left = deserialize(data);
    node.right = deserialize(data);
    return node;
  }
};

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