# Problem 1
# Write three functions that compute the sum of the numbers in a given list using a for-loop, a while-loop, and recursion.

array = [1, 2, 3, 4, 5]
result = 0
# array.sum

array.each do |num|
  result += num
end
puts result

while array.empty? == true
  result = result +=array[0]
end
puts result

def sum_rec(array)
  return 0 if array.length == 0
  array[0] + sum_rec(array[1..-1])
end

# Problem 2
# Write a function that combines two lists by alternatingly taking elements.
#For example: given the two lists [a, b, c] and [1, 2, 3], the function should return [a, 1, b, 2, c, 3].

letters = ['a', 'b', 'c']
numbers = [1, 2, 3]

print letters.zip(numbers).flatten

# Problem 3
# Write a function that computes the list of the first 100 Fibonacci numbers.
# By definition, the first two numbers in the Fibonacci sequence are 0 and 1,
# and each subsequent number is the sum of the previous two. As an example,
# here are the first 10 Fibonnaci numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, and 34.
  a = 0
  b = 1
  10.times do
      temp = a
      a = b
      b = temp + b
  end
puts a

# Problem 4
# Write a function that given a list of non negative integers, arranges them such
#  that they form the largest possible number. For example, given [50, 2, 1, 9],
#  the largest formed number is 95021.

#check [0] index in array for largest number and compare to the rest of the array
#check [-1..1] index in array for largest num

# Problem 5
# Write a program that outputs all possibilities to put + or - or nothing between
# the numbers 1, 2, ..., 9 (in this order) such that the result is always 100.
# For example: 1 + 2 + 34 – 5 + 67 – 8 + 9 = 100.