sum_is_x_two_numbers.rb

a = [1,2,3,4,5]
n = 5

def match_sum_pairs_naive(a,n)
  for i in 0..a.length-2
    for j in i+1..a.length-1
      return [a[i],a[j]] if a[i]+a[j]==n 
    end  
  end  
end

p match_sum_pairs_naive(a,n)

def match_sum_pairs_positive(a,n)
  h = {}
  for i in 0..a.length-1
    h[n-i] = i 
  end  
  for i in 0..a.length-1
    return [a[i],h[a[i]] ]if h[a[i]] 
  end  
end

p match_sum_pairs_positive(a,n)


def match_sum_pairs_sort(a,n)
  b = a.sort
  i,j = 0,a.length-1
  while i<j do
    sum =  a[i] +a[j]
    if sum == n 
      return [a[i],a[j]] 
    elsif sum < n        
      i+=1
    else  
      j-=1
    end  
  end  
end

p match_sum_pairs_sort(a,n)