mergesort.rb

def mergesort(arr)

  return arr if arr.count <= 1

  #Split array in half.
  mid = arr[0..(arr.count / 2) - 1]
  #Left side of array.
  left = arr[0..mid.count - 1]
  #Right side of array.
  right = arr[mid.count..arr.count - 1]

  #Recursive sort on left and right arrays until size of arrays become 1.
  sorted_left = mergesort(left)
  sorted_right = mergesort(right)

  #Merge the sorted array from left and right until there's nothing left to merge.
  sorted_arr = []
  until sorted_left.empty? || sorted_right.empty?
    if sorted_left.first <= sorted_right.first
      #Removes first entry in left array and returns the value.
      sorted_arr << sorted_left.shift
    else
      #Removes first entry in right array and returns the value.
      sorted_arr << sorted_right.shift
    end
  end
  #Add the removed values to the array being rebuilt.
  sorted_arr.concat(sorted_left).concat(sorted_right)
  end

# Find the maximum 
def maximum(arr)
  mergesort(arr).last
end

def minimum(arr)
  mergesort(arr).first
end

# expect it to return 42 below
result = maximum([2, 42, 22, 02])
puts "1. max of 2, 42, 22, 02 is: #{result}"

# expect it to return 2 below
result = minimum([2, 42, 22, 02])
puts "2. min of 2, 42, 22, 02 is: #{result}"
 
# expect it to return nil when empty array is passed in
result = maximum([])
puts "3. max on empty set is: #{result.inspect}"

result = minimum([])
puts "4. min on empty set is: #{result.inspect}"

result = mergesort([1, 10, 5, 78, 2, 39, 29, 55])
puts "5. mergesort of [1, 10, 5, 78, 2, 39, 29, 55] is: #{result}"