A program to determine all pairs of integers in a given array of integers that add to 100. The algorithm is destructive on the original array. This particular algorithm runs in N lg N time and constant space if the sorting algorithm is efficient; after the sort, only one additional pass through the array is required.

hundred_pairs.rb

def hundred_pairs(array, target = 100)
  array.sort!
  results = []
  min, max = array.shift, array.pop
  while min && max
    while min + max > target
      max = array.pop
    end
    while min && max && min + max < target
      min = array.shift
    end
    if min && max && min + max == target
      results << [min,max]
      min, max = array.shift, array.pop
    end
  end
  results
end