Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

Given binary tree {1,#,2,3},

   1    \     2    /   3

/** * Created by JZloveSnow on 16/3/7. */class Solution16 {    public ArrayList<Integer> postorderTraversal(TreeNode root) {        ArrayList<Integer> result = new ArrayList<Integer>();        reverse(root, result);        return result;    }    private void reverse(TreeNode root, ArrayList<Integer> result) {        if (root == null) return;        reverse(root.left, result);        reverse(root.right, result);        result.add(root.val);    }}