/**
 * 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) {
    let list = [];
    const q = [root];
    while (q.length) {
        let p = q.shift();
        list.push(p ? p.val : null);
        if (p) {
            q.push(p.left);
            q.push(p.right);
        }
    }
    return list.join(',');
};

/**
 * Decodes your encoded data to tree.
 *
 * @param {string} data
 * @return {TreeNode}
 */
var deserialize = function(data) {
    data = data.split(',').map(x => parseInt(x));
    if (isNaN(data[0])) return null;
    const root = new TreeNode(data[0]);
    const q = [root];
    let idx = 1;
    while (q.length) {
        let p = q.shift();
        p.left = isNaN(data[idx]) ? null : new TreeNode(data[idx]);
        ++idx;
        if (p.left) q.push(p.left);
        p.right = isNaN(data[idx]) ? null : new TreeNode(data[idx]);
        ++idx;
        if (p.right) q.push(p.right);
    }
    return root;
};