add scala solution for Remove-K-Digits

Author: HangMerlin

May-LeetCoding-Challenge/13-Remove-K-Digits/Remove-K-Digits.scala

@@ -0,0 +1,31 @@
+object Solution {
+  def removeKdigits(num: String, k: Int): String =
+    if (num.length == k) "0"
+    else trimLeftZero(removeKdigitsWithAcc(num, num.length - k, ""))
+
+  def removeKdigitsWithAcc(num: String, restNumberLength: Int, acc: String): String = {
+    if (restNumberLength == 0) acc
+    else if (num.length == restNumberLength) acc + num
+    else {
+      val rightIndexEnd = num.length - restNumberLength
+      var smallestNumberIndex = 0
+      var smallestNumber = num.head
+      var index = 1
+      while (index <= rightIndexEnd && smallestNumber != '0') {
+        if (num(index).toInt < smallestNumber.toInt) {
+          smallestNumber = num(index)
+          smallestNumberIndex = index
+        }
+        index += 1
+      }
+      val newAcc = acc + smallestNumber
+      removeKdigitsWithAcc(num.drop(smallestNumberIndex + 1), restNumberLength - 1, newAcc)
+    }
+  }
+
+  def trimLeftZero(num: String): String = {
+    val leftZeroDroppedNumber = num.dropWhile(_ == '0')
+    if (leftZeroDroppedNumber.isEmpty) "0" else leftZeroDroppedNumber
+  }
+
+}