Binary Tree Postorder Traversal

Difficulty

Easy

Question

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

Example 1:

Input: root = [1,null,2,3]
Output: [3,2,1]

Example 2:

Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
Output: [4,6,7,5,2,9,8,3,1]

Example 3:

Input: root = []
Output: []

Example 4:

Input: root = [1]
Output: [1]

Constraints:

The number of the nodes in the tree is in the range `[0, 100]`.
`-100 <= Node.val <= 100`

Follow up:

Recursive solution is trivial, could you do it iteratively?

Link

Binary Tree Postorder Traversal

Solution

Solution uses two stacks to traverse the tree in desired order.