                    ABCD
                    CABD
                    ACDB
                    DACB
                    BCDA
                    ACBD
                    ADCB
                    CDAB
                    DABC
                    BCAD
                    CADB
                    CDBA
                    CBAD
                    ABDC
                    ADBC
                    BDCA
                    DCBA
                    BACD
                    BADC
                    BDAC
                    CBDA
                    DBCA
                    DCAB

Listed above are   all-but-one   of the permutations of the symbols   A,   B,   C,   and   D,   except   for one permutation that's   not   listed.



Task

Find that missing permutation.



Methods

 Obvious method: 
        enumerate all permutations of   A,  B,  C,  and  D,  
        and then look for the missing permutation. 

 alternate method:
        Hint:  if all permutations were shown above,  how many 
        times would  A  appear in each position?     
        What is the  parity  of this number?

 another alternate method:
        Hint:  if you add up the letter values of each column, 
        does a missing letter   A,  B,  C,  and  D   from each
        column cause the total value for each column to be unique?



Related task

   Permutations)


import "./set" for Set
import "./perm" for Perm
 
var missingPerms = Fn.new { |input, perms|
    var s1 = Set.new()
    s1.addAll(perms)
    var perms2 = Perm.list(input).map { |p| p.join() }
    var s2 = Set.new()
    s2.addAll(perms2)
    return s2.except(s1).toList
}
 
var input = ["A", "B", "C", "D"]
var perms = [
    "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",
    "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",
    "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"
]
var missing = missingPerms.call(input, perms)
if (missing.count == 1) {
    System.print("The missing permutation is %(missing[0])")
} else {
    System.print("There are %(missing.count) missing permutations, namely:\n")
    System.print(missing)
}