Find pairs of numbers that add up to the sum (three different methods)

pairs.rb

# runtime O(N^2)
# memory O(1)

def find_pair(numbers, sum)
    numbers.each_with_index do |n, i|
        numbers.each_with_index do |m, j|
            if n + m == sum
                return [n, m]
            end
        end
    end
    return nil
end



# runtime O(N)
# memory O(N)

def find_pair(numbers, sum)
    hash = {}
    numbers.each_with_index do |n, i|
        hash[n] = n
    end
    numbers.each_with_index do |n, i|
        if hash.has_key?(sum-n)
          return [n, sum-n]
        end
    end

    return nil
end



# Assumption: Sorted numbers
# runtime O(N)
# memory O(1)

def find_pair(numbers, sum)
    i = 0
    j = numbers.length-1

    while j >= 0 && i < numbers.length
        if numbers[i] + numbers[j] > sum
            j -= 1
        elsif numbers[i] + numbers[j] < sum
            i += 1
        else
            return [ numbers[i], numbers[j] ]
        end
    end

    return nil
end