PS script for create dirt tree rows for my semestral thesis

CreateDirtTreeRows.ps1

function CreateDirtTree($folderRoot, $level)
{
    if(-not $(Test-Path $folderRoot))
    {
        Write-Error "Path do not exist"
        return
    }  
    if(-not $level){
        $level = 1
    }
    $items = New-Object System.Collections.ArrayList;
    $childItems = ls -Path $folderRoot
    foreach ($item in $childItems) {
        $size = [math]::Round($($item.length/1KB))
        if($size -eq 0){
            $size = 1
        }

        if($item -is [System.IO.DirectoryInfo]){
            $items +=  ("`t" * $($level-1)) + ".$level "+ $item.Name + "."
            $dirItems = CreateDirtTree -folderRoot $item.FullName -level $($level + 1)
            $dirItems | %{ $items += $_ } 
        }
        else{
            $items +=  ("`t" * $level) + ".$level "+ $item.Name +".\DTcomment{" + $size + " KB}."
        }
    }
    return $items
}