Sorted Merge (https://github.com/turingschool/challenges/blob/d7f7d3e726ce95667ed3f1e1996de2895efadc7c/sorted_merging.markdown)

sorted_merging.rb

# https://github.com/turingschool/challenges/blob/d7f7d3e726ce95667ed3f1e1996de2895efadc7c/sorted_merging.markdown
# I accidentally closed the window I wrote the solution in,
# so I couldn't find it, but I rewrote it. I shortened the names,
# so that I could get some of the ideas downt o one line.
# Also, thought of a way cooler way to implement our `smallest` method :)

def smallest(one, two)
  [one, two].min_by(&:first)
end

def merge(one, two)
  sorted = []
  sorted << smallest(one, two).shift while one.any? && two.any?
  sorted << one.shift                while one.any?
  sorted << two.shift                while two.any?
  sorted
end

merge([],      [])      # => []
merge([],      [1])     # => [1]
merge([1],     [])      # => [1]
merge([1,2],   [])      # => [1, 2]
merge([],      [1,2])   # => [1, 2]
merge([1],     [2])     # => [1, 2]
merge([2],     [1])     # => [1, 2]
merge([1,3],   [2])     # => [1, 2, 3]
merge([2],     [1,3])   # => [1, 2, 3]
merge([1,2,3], [4])     # => [1, 2, 3, 4]
merge([4],     [1,2,3]) # => [1, 2, 3, 4]

sorted_merging_recursive.rb

# Someone had asked about solving it with recursion,
# so here is a solution using recursion :)

def smallest(one, two)
  [one, two].min_by(&:first)
end

def recursive_merge(one, two)
  return two if one.empty?
  return one if two.empty?  
  [smallest(one, two).shift, *recursive_merge(one, two)]
end

recursive_merge([],      [])      # => []
recursive_merge([],      [1])     # => [1]
recursive_merge([1],     [])      # => [1]
recursive_merge([1,2],   [])      # => [1, 2]
recursive_merge([],      [1,2])   # => [1, 2]
recursive_merge([1],     [2])     # => [1, 2]
recursive_merge([2],     [1])     # => [1, 2]
recursive_merge([1,3],   [2])     # => [1, 2, 3]
recursive_merge([2],     [1,3])   # => [1, 2, 3]
recursive_merge([1,2,3], [4])     # => [1, 2, 3, 4]
recursive_merge([4],     [1,2,3]) # => [1, 2, 3, 4]

sorted_merging_recursive_2.rb

def recursive_merge2(*arrays)
  arrays.reject! &:empty?
  arrays.empty? ? [] : [arrays.min_by(&:first).shift, *recursive_merge2(*arrays)]
end

recursive_merge2([],      [])      # => []
recursive_merge2([],      [1])     # => [1]
recursive_merge2([1],     [])      # => [1]
recursive_merge2([1,2],   [])      # => [1, 2]
recursive_merge2([],      [1,2])   # => [1, 2]
recursive_merge2([1],     [2])     # => [1, 2]
recursive_merge2([2],     [1])     # => [1, 2]
recursive_merge2([1,3],   [2])     # => [1, 2, 3]
recursive_merge2([2],     [1,3])   # => [1, 2, 3]
recursive_merge2([1,2,3], [4])     # => [1, 2, 3, 4]
recursive_merge2([4],     [1,2,3]) # => [1, 2, 3, 4]