why my merge sort not working?

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PowerShell

I changed a C# merge sort function into powershell, but it's not working anymore. There are a lot of random numbers added to the result. Could anyone tell what's wrong? Thanks so much!

$list = 1,5,9,7,2,1,3,20, 3,8,5,1,9,5,6,4,200

function mergeSort ([System.Collections.ArrayList] $m)
{
    $result = New-Object System.Collections.ArrayList
    $left = New-Object System.Collections.ArrayList
    $right = New-Object System.Collections.ArrayList

    if($m.Count -le 1)
    {
        return $m
    }

    [int]$middle = $m.Count / 2
    for($i = 0; $i -lt $middle; $i++)
    {
        $left.Add($m[$i])
    }
    for($i = $middle; $i -lt $m.Count; $i++)
    {
        $right.Add($m[$i])
    }

    $left =  mergeSort $left
    $right = mergeSort $right

    if ($left[$($left.Count - 1)] -le $right[0])
    {
        return append $left $right
    }

    $result = merge $left $right
    return $result
}

function append ([System.Collections.ArrayList] $a, [System.Collections.ArrayList] $b)
{
    $s = New-Object System.Collections.ArrayList
    $s = [System.Collections.ArrayList] $a + [System.Collections.ArrayList] $b
    return $s
}


function merge ([System.Collections.ArrayList] $a, [System.Collections.ArrayList] $b)
{
    $s = New-Object System.Collections.ArrayList
    while($a.Count -gt 0 -and $b.Count -gt 0)
    {
        if($a[0] -lt $b[0])
        {
            $s.Add($a[0])
            $a.RemoveAt(0)
        }
        else
        {
            $s.Add($b[0])
            $b.RemoveAt(0)
        }
    }

    while($a.Count -gt 0)
    {
        $s.Add($a[0])
        $a.RemoveAt(0)
    }
    while($b.Count -gt 0)
    {
        $s.Add($b[0])
        $b.RemoveAt(0)
    }

    return $s
}

$newlist = mergeSort $list