def find(l,aim,start = 0,end = None):    end = len(l) if end is None else end    mid_index = (end - start)//2 + start    if start <= end:        if l[mid_index] < aim:            return find(l,aim,start =mid_index+1,end=end)        elif l[mid_index] > aim:            return find(l, aim, start=start, end=mid_index-1)        else:            return mid_index    else:        return '找不到这个值'