后序

function postorderTraversal( root ) {
    function postOrder(root){
      if(root == null)  return;
      postOrder(root.left);
      postOrder(root.right);
      res.push(root.val);
    }
    let res = [];
    postOrder(root);
    return res;
}
module.exports = {
    postorderTraversal : postorderTraversal
};