Solution on Kotlin for https://www.facebook.com/Engineering/videos/10152735777427200/

NearbyWords.kt

package NearbyWords

import kotlin.collections.HashSet

fun main(args : Array<String>) {
    println(nearbyWords("gi"))
}

fun nearbyWords(word: String): Set<String> {
    val permutations = getAllWordPermutations(word, 0)
    return permutations.filter { isWord(it) }.toSet()
}

fun getAllWordPermutations(word: String, index: Int): Set<String> {
    if(index >= word.length)
        return setOf("")

    val sub_words = getAllWordPermutations(word, index+1)
    val nearby_letters = getNearbyChar(word[index])
    val permutations = HashSet<String>()

    sub_words.forEach { sub_word ->
        nearby_letters.forEach {
            permutations.add("$it$sub_word")
        }
    }

    return permutations
}

//simulating helper function
fun getNearbyChar(char: Char): Set<Char> {
    return HashSet<Char>().apply {
        when(char) {
            'g' -> addAll(arrayListOf('g', 'h', 'f'))
            'i' -> addAll(arrayListOf('i', 'o', 'k'))
        }
    }
}

//simulating helper function
fun isWord(word: String) : Boolean {    
  return (word == "go" || word == "hi")
}