Serialize / Deserialize

You can do in this in many ways, the following code does this using prefix traversal.

/**
 * @param {Node} root
 * @return {string}
 */
function serialize(root) {
  if(!root) return '_';
  return `${root.val},${serialize(root.left)},${serialize(root.right)}`;
}

/**
 * @param {string} str
 * @return {Node}
 */
function deserialize(str) {
  const q = str.split(',');
  return dfs(q);

 /**
 * @param {string} str
 * @return {Node}
 */
  function dfs(que) {
    if(!que.length) return null;
    const n = que.shift()
;
    if(n !== '_') {
      const node = new Node(n.value)
      node.left = dfs(q);
      node.right = dfs(q);
      return node;
    }
    return null;
  }
}