鸡尾酒排序代码中，已经排序好的不用再遍历了

def cocktail_sort!
    f  = 0
    while f < self.length/2
        i = f
        while i < self.length - 1 - f
            self[i], self[i+1] = self[i+1], self[i] if self[i] > self[i+1]
            i += 1;
        end
        t = self.length - 1 - f
        while t > f
            self[t], self[t-1] = self[t-1], self[t] if self[t] < self[t-1]
            t -= 1
        end
        f += 1
    end
    self
end