Code Golf: Numeric Ranges

Answer 1

Ruby : 123 characters

def y(n) t=[];r=[];n.each_with_index do |x,i| t<2)?r<

More Readable

def y(n) 
t=[];r=[];
n.each_with_index do |x,i|
 t << x
 if (x.succ != n[i+1])
    r = ((t.size > 2) ? r << t[0] << -t[-1] : r+t)  
    t=[]
 end
 end
 r
end

And execute like

 > n=[1, 2, 3, 4, 7, 8, 10, 12, 13, 14, 15]
 > y n
 => [1, -4, 7, 8, 10, 12, -15]