中位数

#encoding=utf-8
class Solutiob(object):
    def midNum(self,arr):
        if not arr:#判断arr是空
            print "nothing"
            return
        length = len (arr)
        if length==1:
            return arr[0]
        n = length//2
        if length%2==1:
            return arr[n]#下标是n不是n+1
        else:
            return (arr[n]+arr[n-1])/2.0
if __name__=="__main__":
    A  = Solutiob()
    result = A.midNum([])
    print result