# iterative
function traverse($tree) {
    $stack = [System.Collections.Stack]::new()
    $stack.Push($tree)
    while ($stack.Count) {
        $node = $stack.Pop()
        if ($node.Left) {
            $stack.Push($node.Left)
        }
        if ($node.Right) {
            $stack.Push($node.Right)
        }
        ... # do something with the node
    }
}

# recursive
function traverse($tree)
    $tree.left, $tree.right | ? {$_} | % {traverse $_}
    ... # do something with the node
}