94. Binary Tree Inorder Traversal (copy).js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */

var preorderTraversal = function(root) {
    if(root == null) return [];
        const arr = [];
        function traverse(root){
                if(root == null) {
                        return;
                }
                arr.push(root.val);
                traverse(root.left);
                traverse(root.right);
        }        
        traverse(root);
        return arr;
};