problems: add „1286. Iterator for Combination“

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/iterator-for-combination.kt

@@ -0,0 +1,46 @@
+class CombinationIterator(val characters: String, val combinationLength: Int) {
+    private var indices = (0 until combinationLength).toMutableList()
+
+    private fun nextCombination(index: Int) {
+        // bump the current index
+        indices[index]++
+
+        // we have gone through the whole string
+        while (indices[index] == characters.length && index > 0) {
+            // bump the previous index
+            nextCombination(index - 1)
+
+            // set current index to previous + 1, since it must be the following one
+            indices[index] = indices[index - 1] + 1
+        }
+    }
+
+    // By default bumping just the last index
+    private fun nextCombination() = nextCombination(combinationLength - 1)
+
+    private fun getCombination(): String =
+        indices.map { characters[it] }.joinToString(separator="")
+
+    fun next(): String {
+        // construct the combination
+        val combination = getCombination()
+
+        // bump the combination
+        nextCombination()
+
+        return combination
+    }
+
+    fun hasNext(): Boolean =
+        indices.all {
+            it < characters.length
+        }
+
+}
+
+/**
+ * Your CombinationIterator object will be instantiated and called as such:
+ * var obj = CombinationIterator(characters, combinationLength)
+ * var param_1 = obj.next()
+ * var param_2 = obj.hasNext()
+ */