if m1 > m2 then median must exist in ar1 [ ... . m1 ] and ar2 [ m2 ... ]
if ( n % 2 == 0 ) return getMedian ( ar2 + n / 2 - 1 , ar1 , n - n / 2 + 1 ) ; return getMedian ( ar2 + n / 2 , ar1 , n - n / 2 ) ; }