2019-03-22-n-ary tree postorder traversal

var postorder = function(root) {
  const res = [];
  if(!root) {
    return [];
  }
  for(let i=0; i<root.children.length; i++) {
    res.push(...postorder(root.children[i]));
  }
  res.push(root.val);
  return res;
};