Merge pull request #1605 from t3chkid/main

Adds solutions for problems 17 & 51 in Kotlin

Author: a93a

Diff for: kotlin/0017-letter-combinations-of-a-phone-number.kt

@@ -0,0 +1,34 @@
+package kotlin
+
+class Solution {
+    fun letterCombinations(digits: String): List<String> {
+        if (digits.isEmpty()) return emptyList()
+        val resultantList = mutableListOf<String>()
+        val stringBuilder = StringBuilder()
+        val digitCharListMapping = mutableMapOf(
+            '2' to listOf('a', 'b', 'c'),
+            '3' to listOf('d', 'e', 'f'),
+            '4' to listOf('g', 'h', 'i'),
+            '5' to listOf('j', 'k', 'l'),
+            '6' to listOf('m', 'n', 'o'),
+            '7' to listOf('p', 'q', 'r', 's'),
+            '8' to listOf('t', 'u', 'v'),
+            '9' to listOf('w', 'x', 'y', 'z')
+        )
+
+        fun dfs(decisionIndex: Int = 0) {
+            if (stringBuilder.length == digits.length) {
+                resultantList.add(stringBuilder.toString())
+                return
+            }
+            val charListForDigitAtDecisionIndex = digitCharListMapping.getValue(digits[decisionIndex])
+            for (char in charListForDigitAtDecisionIndex) {
+                stringBuilder.append(char)
+                dfs(decisionIndex + 1)
+                stringBuilder.deleteCharAt(stringBuilder.lastIndex)
+            }
+        }
+        dfs()
+        return resultantList
+    }
+}