interview_t9.cs

private static Hashset<string> ValidWords;       
public static IEnumerable<string> TranslateEntry(string input)
{
    var dialpad = new[] {
        new char[0],
        new char[0],
        new[]{'a','b','c'},
        new[]{'d','e','f'},
        new[]{'g','h','i'},
        new[]{'j','k','l'},
        new[]{'p','q','r','s'},
        new[]{'t','u','v'},
        new[]{'w','x','y','z'}
    };

    var candidates = input.Aggregate(new[] { "" }.AsEnumerable(), (results, elem) => results.SelectMany(c => dialpad[int.Parse(elem.ToString())].Select(e => c + e)));
    return ValidWords.Intersect(candidates);
}

interview_T9.py

from typing import List
import functools
T9 = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
def T9Words(word:int)->List[str]:
    return functools.reduce(lambda results,elem: [r+d for r in results for d in T9[int(elem)]],str(word),[''])

interview_t9.sql

create or alter function T9Words(@input int)
returns table
with schemabinding as return
with t9 as (
    select i,d
    from (values
         (2,'a'),(2,'b'),(2,'c')
        ,(3,'d'),(3,'e'),(3,'f')
        ,(4,'g'),(4,'h'),(4,'i')
        ,(5,'j'),(5,'k'),(5,'l')
        ,(6,'m'),(6,'n'),(6,'o')
        ,(7,'p'),(7,'q'),(7,'r'),(7,'s')
        ,(8,'t'),(8,'u'),(8,'v')
        ,(9,'w'),(9,'x'),(9,'y'),(9,'z')) f(i,d))
   ,rec as (
      select rem=@input,res=convert(varchar(8000),'')
      union all
      select rem=rem/10,res=d+res
      from rec
      join t9 z
        on z.i = rem%10
      where rem != 0) 
select res
from rec
where rem=0