September 2017 Update

Task/Find-the-missing-permutation/Kotlin/find-the-missing-permutation.kotlin

@@ -0,0 +1,34 @@
+// version 1.1.2
+
+fun <T> permute(input: List<T>): List<List<T>> {
+    if (input.size == 1) return listOf(input)
+    val perms = mutableListOf<List<T>>()
+    val toInsert = input[0]
+    for (perm in permute(input.drop(1))) {
+        for (i in 0..perm.size) {
+            val newPerm = perm.toMutableList()
+            newPerm.add(i, toInsert)
+            perms.add(newPerm)
+        }
+    }
+    return perms
+}
+
+fun <T> missingPerms(input: List<T>, perms: List<List<T>>) = permute(input) - perms
+
+fun main(args: Array<String>) {
+    val input = listOf('A', 'B', 'C', 'D')
+    val strings = listOf(
+        "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",
+        "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",
+        "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"
+    )
+    val perms = strings.map { it.toList() }
+    val missing = missingPerms(input, perms)
+    if (missing.size == 1)
+        print("The missing permutation is ${missing[0].joinToString("")}")
+    else {
+        println("There are ${missing.size} missing permutations, namely:\n")
+        for (perm in missing) println(perm.joinToString(""))
+    }
+}