/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: A Tree     * @return: Postorder in ArrayList which contains node values.     */   List
     
       list = new ArrayList<>();     public List
      
        postorderTraversal(TreeNode root) {         hou(root);         return list;        }     public void hou(TreeNode root) {         if(root == null) {             return;         }         hou(root.left);         hou(root.right);         list.add(root.val);     }}