Data commit

Task/Find-the-missing-permutation/CoffeeScript/find-the-missing-permutation.coffee

@@ -0,0 +1,35 @@
+missing_permutation = (arr) ->
+  # Find the missing permutation in an array of N! - 1 permutations.
+
+  # We won't validate every precondition, but we do have some basic
+  # guards.
+  if arr.length == 0
+    throw Error "Need more data"
+  if arr.length == 1
+      return [arr[0][1] + arr[0][0]]
+
+  # Now we know that for each position in the string, elements should appear
+  # an even number of times (N-1 >= 2).  We can use a set to detect the element appearing
+  # an odd number of times.  Detect odd occurrences by toggling admission/expulsion
+  # to and from the set for each value encountered.  At the end of each pass one element
+  # will remain in the set.
+  result = ''
+  for pos in [0...arr[0].length]
+      set = {}
+      for permutation in arr
+          c = permutation[pos]
+          if set[c]
+            delete set[c]
+          else
+            set[c] = true
+      for c of set
+        result += c
+        break
+  result
+
+given = '''ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA
+  CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB'''
+
+arr = (s for s in given.replace('\n', ' ').split ' ' when s != '')
+
+console.log missing_permutation(arr)