solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} t
 * @return {string}
 */
var tree2str = function(t) {
    if (t === null) {
        return ''
    } else if (t.left === null && t.right === null) {
        return `${t.val}`
    } else if (t.right === null) {
        return `${t.val}(${tree2str(t.left)})`
    } else {
         return `${t.val}(${tree2str(t.left)})(${tree2str(t.right)})`
    }
};