Postorder Traversal of a Binary Tree using Iteration

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def postorderTraversal(root):
    if not root:
        return []
    stack1, stack2 = [root], []
    while stack1:
        node = stack1.pop()
        stack2.append(node)
        if node.left:
            stack1.append(node.left)
        if node.right:
            stack1.append(node.right)
    result = []
    while stack2:
        result.append(stack2.pop().val)
    return result

# Example usage:
if __name__ == '__main__':
    # Construct binary tree:
    #        1
    #       / \
    #      2   3
    #     / \
    #    4   5
    root = TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3))
    print(postorderTraversal(root))