Same as ( low + high ) / 2 , but avoids overflow for large low and high
let mid = low + Math . floor ( ( high - low ) / 2 ) ; let str1 = commonPrefix ( arr , low , mid ) ; let str2 = commonPrefix ( arr , mid + 1 , high ) ; return ( commonPrefixUtil ( str1 , str2 ) ) ; } return null ; }