Five programming problems every software engineer should be able to solve in less than 1 hour

problem5.rb

# https://www.shiftedup.com/2015/05/07/five-programming-problems-every-software-engineer-should-be-able-to-solve-in-less-than-1-hour
#
# 5th problem:

# 1 _ 23456789                        1 op
# 1 _ 2 _ 3456789                     2 ops
# 1 _ 23 _ 456 _ 789                  3 ops
# ...
# 1 _ 2 _ 3 _ 4 _ 5 _ 6 _ 7 _ 8 _ 9   8 ops
#
# 1 op combinations:        | Op permutations +,-
# 1 _ 23456789 (op after 1) | 1 + 23456789
#                           | 1 - 23456789
#
# 12 _ 3456789 (op after 2) | 12 + 3456789
#                           | 12 - 3456789
#
# ...
#
# 2 op combinations:                | Op permutations ++, +-, -+, --
# 1 _ 2 _ 3456789 op after 1 and 2  | 1 + 2 + 3456789
#                                   | 1 + 2 - 3456789
#                                   | 1 - 2 + 3456789
#                                   | 1 - 2 - 3456789
#
# 1 _ 23 _ 456789 op after 1 and 3  | 1 + 23 + 456789
#                                   | 1 + 23 - 456789
#                                   | 1 - 23 + 456789
#                                   | 1 - 23 - 456789
#
# Left: operation position, Right: what operation goes into each position
#
def plus_minus
  (1..8).each do |number_of_ops|
    positions_combinations  = (1..8)  .to_a.combination     number_of_ops
    operations_permutations = %w(+ -) .repeated_permutation number_of_ops

    positions_combinations.each do |position_combination|
      operations_permutations.each do |operations|
        expr = "123456789"

        position_combination.each_with_index do |position, index|
          expr.gsub! /#{position}/, "#{position}#{operations[index]}"
        end

        puts expr if eval(expr) == 100
      end
    end
  end
end

problems_1_to_4.rb

def sum_for(list)
  sum = 0

  for i in 0..(list.size - 1)
    sum += list[i]
  end

  sum
end

def sum_while(list)
  sum, i = 0, 0

  while i < list.size
    sum += list[i]
    i += 1
  end

  sum
end

def sum_recursive(list)
  return 0 if list.size == 0
  list[0] + sum_recursive(list[1..-1])
end

tests = [
  [],
  [1],
  [1,2],
  [1,2,3]
]

tests.each do |list|
  puts "List #{list}"
  puts "For #{sum_for list}"
  puts "While #{sum_while list}"
  puts "Recursive #{sum_recursive list}"
  puts
end

def combine(a, b)
  a.map.with_index {|x, i| [x, b[i] ] }.flatten
end


puts "Combine #{combine [:a, :b, :c], [1, 2, 3]}"

def fibonacci_100
  x = [0, 1]
  while x.size != 100
    x << x[-1] + x[-2]
  end

  puts "Fibonacci (#{x.size}): #{x}"
end

fibonacci_100

def largest_permutation_number(list)
  max, winner = 0, nil

  list.permutation(list.size).each do |permut|
    value = permut.join.to_i
    max, winner = value, permut if value >= max
  end

  [max, winner]
end

puts "Largest permut number: #{largest_permutation_number [50, 2, 1, 9]}"