Leetcode | Letter Combinations of a Phone Number | Kotlin

devstromo · devstromo · commit 39905105f1fa · 2025-05-09T21:51:21.000-03:00
diff --git a/leetcode/src/main/kotlin/com/devstromo/medium/p17/DESCRIPTION.md b/leetcode/src/main/kotlin/com/devstromo/medium/p17/DESCRIPTION.md
@@ -0,0 +1,27 @@
+# 17. Letter Combinations of a Phone Number
+
+Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could
+represent. Return the answer in any order.
+
+A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any
+letters.
+
+Example 1:
+
+Input: digits = "23"
+Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+
+Example 2:
+
+Input: digits = ""
+Output: []
+
+Example 3:
+
+Input: digits = "2"
+Output: ["a","b","c"]
+
+Constraints:
+
+- `0 <= digits.length <= 4`
+- `digits[i]` is a digit in the range `['2', '9']`.
diff --git a/leetcode/src/main/kotlin/com/devstromo/medium/p17/SolutionKt.kt b/leetcode/src/main/kotlin/com/devstromo/medium/p17/SolutionKt.kt
@@ -0,0 +1,35 @@
+package com.devstromo.medium.p17
+
+class SolutionKt {
+    private val phoneMap = mapOf(
+        '2' to "abc", '3' to "def",
+        '4' to "ghi", '5' to "jkl", '6' to "mno",
+        '7' to "pqrs", '8' to "tuv", '9' to "wxyz"
+    )
+
+    fun letterCombinations(digits: String): List<String> {
+        if (digits.isEmpty()) return emptyList()
+        val result = mutableListOf<String>()
+        backtrack(digits, 0, StringBuilder(), result)
+        return result
+    }
+
+    private fun backtrack(
+        digits: String,
+        index: Int,
+        path: StringBuilder,
+        result: MutableList<String>
+    ) {
+        if (index == digits.length) {
+            result.add(path.toString())
+            return
+        }
+
+        val letters = phoneMap[digits[index]] ?: return
+        for (c in letters) {
+            path.append(c)
+            backtrack(digits, index + 1, path, result)
+            path.deleteCharAt(path.lastIndex)
+        }
+    }
+}
diff --git a/leetcode/src/test/kotlin/com/devstromo/medium/p17/SolutionKtUnitTest.kt b/leetcode/src/test/kotlin/com/devstromo/medium/p17/SolutionKtUnitTest.kt
@@ -0,0 +1,18 @@
+package com.devstromo.medium.p17
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class SolutionKtUnitTest {
+    private val solution = SolutionKt()
+
+    @Test
+    fun `Test Letter Combinations of a Phone Number`() {
+        assertEquals(
+            listOf("ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"),
+            solution.letterCombinations("23")
+        )
+        assertEquals(listOf<String>(), solution.letterCombinations(""))
+        assertEquals(listOf("a", "b", "c"), solution.letterCombinations("2"))
+    }
+}
