Greplin Challenge Problems

level_1.rb

class String
  def palindrome?
    front = 0
    back = self.length - 1
  
    while front < back do
      return false if self[front] != self[back]
      
      front += 1
      back -= 1
    end
  
    true
  end
end

def longest_palindrome(str)
    
  length = str.length
  
  length.downto(1) do |len|
    positions = length - len
    (0..positions).each do |start|
      substr = str[start, len]
      
      return substr if substr.palindrome?
    end
  end
  
  nil
end

level_2.rb

def is_prime(n)
  
  (2..Math.sqrt(n)).each do |i|
    return false if n % i == 0      
  end
  
  true
end

def prime_fibonacci(n)
  
  prev = 1
  cur = 1
  
  while true do 
   
   temp = prev
   prev = cur
   cur += temp
   
   return cur if cur > n && is_prime(cur) 
  end
  
end

def sum_divisors(n)
  
  sum = 0
  divisor = 2
  
  until n == 1 do
    
    if n % divisor == 0
      sum += divisor
      
      # divide by all products of divisor
      i = 0
      while n % divisor**i == 0 do
        n /= divisor
        i += 1
      end
      
    end
    
    divisor += 1
  end
  
  sum
end

level_3.rb

# number of subsets in list that fulfill
# sum(list[0..n-2]) == list[n - 1], where
# n is the number of elements. 

# precondition: list is sorted
def num_sets(list, pre = [])  
  count = 0
  
  unless pre.empty?
    sum = pre.pop
    total = pre.reduce{|t, x| t + x}
  
    pre.push(sum)
  
    count += 1 if total == sum
  end
  
  list.each_with_index do |x, i|
    count += num_sets(list[i+1..-1], pre + [x])
  end
  
  count
end