Solution for "Letter Combinations of a Phone Number"

Author: Thuy Trinh

build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-  ext.kotlinVersion = '1.1.60'
+  ext.kotlinVersion = '1.2.0'
 
   repositories {
     mavenCentral()

src/main/java/leetcode/lettercombinations/Solution.kt

@@ -0,0 +1,55 @@
+package leetcode.lettercombinations
+
+import java.util.*
+
+class Solution {
+  private val kb = hashMapOf(
+      '1' to "",
+      '2' to "abc",
+      '3' to "def",
+      '4' to "ghi",
+      '5' to "jkl",
+      '6' to "mno",
+      '7' to "pqrs",
+      '8' to "tuv",
+      '9' to "wxyz",
+      '0' to ""
+  )
+
+  fun letterCombinations(digits: String): List<String> {
+    // "abc" and "def"
+    // * Pick 'a'. Push 'a'.
+    // * Pick 'd'. Push 'd'.
+    // * Print stack. Pop stack.
+    // * Pick 'e'. Push 'e'.
+    // * Print stack. Pop stack.
+    return when {
+      digits.isEmpty() -> emptyList()
+      else -> {
+        val texts: List<String> = digits.map { kb[it] ?: "" }
+        val result = mutableListOf<String>()
+        generate(texts, 0, result, Stack())
+        result
+      }
+    }
+  }
+
+  private fun generate(
+      texts: List<String>, // texts = ["abc", "def"]
+      i: Int, // i = 2
+      result: MutableList<String>, // result = []
+      stack: Stack<Char> // stack = [a, e]
+  ) {
+    when (i) { // i = 2
+      texts.size -> { // size = 2
+        // result = ["ad", "ae"]
+        result += stack.joinToString(separator = "")
+      }
+      else -> texts[i].forEach {
+        stack += it // it = 'f', stack = [a, f]
+        generate(texts, i + 1, result, stack)
+        stack.pop() // stack = [a]
+      }
+    }
+  }
+}

src/test/java/leetcode/lettercombinations/SolutionTest.kt

@@ -0,0 +1,19 @@
+package leetcode.lettercombinations
+
+import org.amshove.kluent.shouldBeEmpty
+import org.amshove.kluent.shouldContainAll
+import org.junit.Test
+
+class SolutionTest {
+  @Test
+  fun letterCombinations() {
+    Solution()
+        .letterCombinations("23")
+        .shouldContainAll(listOf(
+            "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"
+        ))
+    Solution()
+        .letterCombinations("01")
+        .shouldBeEmpty()
+  }
+}