# Binary Tree Postorder Traversal

{% embed url="<https://leetcode.com/problems/binary-tree-postorder-traversal/>" %}

{% embed url="<https://www.lintcode.com/problem/68/>" %}

## Solution 1 (Go)

```go
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func postorderTraversal(root *TreeNode) []int {
    res := make([]int, 0)
    postorderHelper(root, &res)
    return res
}

func postorderHelper(root *TreeNode, res *[]int) {
    if root != nil {
        postorderHelper(root.Left, res)
        postorderHelper(root.Right, res)
        *res = append(*res, root.Val)
    }
}
```

## Solution 2 (Java)

```java
/**
 * 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.
     */
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        postorderTraversalHelper(root, result);
        return result;
    }

    private void postorderTraversalHelper(TreeNode root, List<Integer> result) {
        if (root == null) return;
        postorderTraversalHelper(root.left, result);
        postorderTraversalHelper(root.right, result);
        result.add(root.val);
    }
}
```

### Notes

* Recursion / Divide and conquer&#x20;

## Solution 3 (Java)

```java
/**
 * 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.
     */
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root == null) return result;
        TreeNode curr = root;
        TreeNode prev = null;
        stack.push(root);

        while (!stack.isEmpty()) {
            curr = stack.peek();
            // traversal down the tree
            if (prev == null || prev.left == curr || prev.right == curr) {
                if (curr.left != null) {
                    stack.push(curr.left);
                } else if (curr.right != null) {
                    stack.push(curr.right);
                }
            } else if (curr.left == prev) {
                // if the left is visited, visit the right
                if (curr.right != null) {
                    stack.push(curr.right);
                }
            } else { // curr.right == prev
                // if the right is visited, visit the middle
                result.add(curr.val);
                stack.pop();
            }
            prev = curr;
        }

        return result;
    }
}
```

### Notes

* Traversal


---

# 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://blog.yushunchen.com/algo/binary-tree/1.-traversal/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.