LeetCode_145Binary Tree Postorder Traversal

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

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [3,2,1].

 private void resPreorder(TreeNode root){         if(root == null) return;         resPreorder(root.left);         resPreorder(root.right);         result.add(root.val);     }     List<Integer> result = new ArrayList<Integer>();     public List<Integer> postorderTraversal(TreeNode root) {         resPreorder(root);         return result;         }