sub TwoSum
{
	my( $target, @nums ) = @_;
	# assumption: @nums are already sorted.
	# caveat: the nums in the list might not be uniq!!!
	
	my %num; @num{@nums} = (); # for existence tests.

	for my $i2 ( reverse( 1 .. $#nums ) )
	{
		my $d = $target - $nums[$i2];
		if ( exists $num{$d} )
		{ # gotta find the _first_ occurrence of this number in the list:
			my($i1) = grep { $nums[$_] == $d } 0 .. $i2-1;
			return( $i1, $i2 );
		}
	}

	die
}