function Test-Sum
{
    param(
        [Parameter( Mandatory, Position = 0 )]
        [Int[]]
        $List,

        [Parameter( Mandatory, Position = 1 )]
        [Int]
        $Target
    )

    Write-Host "Checking List of $($List.count) numbers for a sum of $Target"
    $visited = @{}

    for ( $i = 0; $i -lt $List.count; $i++ )
    {
        $number = $List[$i]

        if ( $null -ne $visited[$Target - $number] )
            {
                Write-Output "$($Target - $number) + $number = $Target"
                return $true
            }
            $visited[$number] = $Target - $number
        }
    return $false
}

Test-Sum -List -10,2,3,4,20,35 25
Test-Sum -List 0,2,3,4,20,25,35 25

2000,4000,8000,16000 | % {
    "in $((Measure-Command {Test-Sum @(1..$_) $_}).TotalMilliseconds) ms"
}