1
var serialize = function (root) {
2
  if (!root) return "";
3
  let res = [];
4

5
  function getNode(node) {
6
    if (!node) {
7
      res.push("null");
8
    } else {
9
      res.push(node.val);
10
      getNode(node.left);
11
      getNode(node.right);
12
    }
13
  }
14

15
  getNode(root);
16

17
  return res.join(",");
18
};
19

20
/**
21
 * Decodes your encoded data to tree.
22
 *
23
 * @param {string} data
24
 * @return {TreeNode}
25
 */
26
var deserialize = function (data) {
27
  if (data === "") return null;
28
  const arr = data.split(",");
29

30
  function buildTree(array) {
31
    const nodeVal = array.shift();
32

33
    if (nodeVal === "null") return null;
34

35
    const node = new TreeNode(nodeVal);
36
    node.left = buildTree(array); //build left first
37
    node.right = buildTree(array); //build right with updated array.
38

39
    return node;
40
  }
41

42
  return buildTree(arr);
43
};