thre_sum_problem.rb

# check if there is three integers which sum up to the targeted sum
# fix k from 0...size-2 and then run sum of 2 numbers equal to target (see above)
def target_sum(arr, target)
  arr.sort!
  size = arr.size
  i, j = 0, size-1
  count = 0
  (0...size-2).each do |k|
    i=k+1
    j=size-1  
    while(i<j) do
      sum = arr[i] + arr[j] + arr[k]
      if sum == target
    	  count += 1
    	  i+=1
      elsif sum < target
    	  i += 1
      else
    	  j -= 1
      end
    end
  end
  count
end
puts target_sum([0,1,2,3,4,5], 5)