tasks a-s

Task/Permutations/Ruby/permutations-1.rb

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+p [1,2,3].permutation.to_a

Task/Permutations/Ruby/permutations-2.rb

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+class Array
+  # Yields distinct permutations of _self_ to the block.
+  # This method requires that all array elements be Comparable.
+  def distinct_permutation  # :yields: _ary_
+    # If no block, return an enumerator. Works with Ruby 1.8.7.
+    block_given? or return enum_for(:distinct_permutation)
+
+    copy = self.sort
+    yield copy.dup
+    return if size < 2
+
+    while true
+      # from: "The Art of Computer Programming" by Donald Knuth
+      j = size - 2;
+      j -= 1 while j > 0 && copy[j] >= copy[j+1]
+      if copy[j] < copy[j+1]
+        l = size - 1
+        l -= 1 while copy[j] >= copy[l]
+        copy[j] , copy[l] = copy[l] , copy[j]
+        copy[j+1..-1] = copy[j+1..-1].reverse
+        yield copy.dup
+      else
+        break
+      end
+    end
+  end
+end
+
+permutations = []
+[1,1,2].distinct_permutation do |p| permutations << p end
+p permutations
+# => [[1, 1, 2], [1, 2, 1], [2, 1, 1]]
+
+if RUBY_VERSION >= "1.8.7"
+  p [1,1,2].distinct_permutation.to_a
+  # => [[1, 1, 2], [1, 2, 1], [2, 1, 1]]
+end