# 145. Binary Tree Postorder Traversal

<https://leetcode.com/problems/binary-tree-postorder-traversal/>

iterative

tricky way: like do preorder to do, but the seq of postorder is not correct, but the answer is correct

```java
class Solution {
    // left right root: postorder
    // so do root right left (like preorder) then reverse(add to res first)
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> res = new LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        while (root != null || !stack.isEmpty()) {
            if (root != null) {
                stack.push(root);
                res.addFirst(root.val);
                root = root.right;
            } else {
                root = stack.pop().left;
            }   
        }
        return res;
    }
}
```

## recursion

```java
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }
    private void helper (TreeNode root, List<Integer> res) {
        if (root == null) return;
        helper(root.left, res);
        helper(root.right, res);
        res.add(root.val);
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://timmybeeflin.gitbook.io/cracking-leetcode/tree/145.-binary-tree-postorder-traversal.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.