feat: add kotlin solution to lc problem: No.0415

Author: Zuoqiu-Yingyi

solution/0400-0499/0415.Add Strings/README.md

@@ -377,6 +377,32 @@ var subStrings = function (num1, num2) {
 };
 ```
 
+#### Kotlin
+
+```kotlin
+class Solution {
+    fun addStrings(num1: String, num2: String): String {
+        val result = mutableListOf<Int>()
+
+        val chars_1 = num1.toCharArray()
+        val chars_2 = num2.toCharArray()
+        var over = 0
+        var i = num1.length
+        var j = num2.length
+
+        while (i > 0 || j > 0 || over > 0) {
+            val a = if (i > 0) chars_1[--i] - '0' else 0
+            val b = if (j > 0) chars_2[--j] - '0' else 0
+            val sum = a + b + over
+            over = sum / 10
+            result.add(sum % 10)
+        }
+
+        return result.reversed().joinToString("")
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

solution/0400-0499/0415.Add Strings/README_EN.md

@@ -373,6 +373,32 @@ var subStrings = function (num1, num2) {
 };
 ```
 
+#### Kotlin
+
+```kotlin
+class Solution {
+    fun addStrings(num1: String, num2: String): String {
+        val result = mutableListOf<Int>()
+
+        val chars_1 = num1.toCharArray()
+        val chars_2 = num2.toCharArray()
+        var over = 0
+        var i = num1.length
+        var j = num2.length
+
+        while (i > 0 || j > 0 || over > 0) {
+            val a = if (i > 0) chars_1[--i] - '0' else 0
+            val b = if (j > 0) chars_2[--j] - '0' else 0
+            val sum = a + b + over
+            over = sum / 10
+            result.add(sum % 10)
+        }
+
+        return result.reversed().joinToString("")
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

solution/0400-0499/0415.Add Strings/Solution.kt

@@ -0,0 +1,21 @@
+class Solution {
+    fun addStrings(num1: String, num2: String): String {
+        val result = mutableListOf<Int>()
+
+        val chars_1 = num1.toCharArray()
+        val chars_2 = num2.toCharArray()
+        var over = 0
+        var i = num1.length
+        var j = num2.length
+
+        while (i > 0 || j > 0 || over > 0) {
+            val a = if (i > 0) chars_1[--i] - '0' else 0
+            val b = if (j > 0) chars_2[--j] - '0' else 0
+            val sum = a + b + over
+            over = sum / 10
+            result.add(sum % 10)
+        }
+
+        return result.reversed().joinToString("")
+    }
+}