def largest(lst):
    """Returns the largest element of lst, the same as max(lst).
        Precondition: lst is not empty."""
    if lst==[]:
        return "error"
    elif len(lst)==1:
        return lst[0]
    else:
        halfLst=len(lst)//2
        Llst=lst[:halfLst]
        Rlst=lst[halfLst:]
        return max(largest(Llst), largest(Rlst))


def largest2(lst,start=0,end=None):
    """Returns the largest element of lst[start:end].
        Precondition: end >= start."""
    if end == None:
        end = len(lst)
    if start == end:
        return "error"
    elif end - start == 1:
        return lst[start]
    else:
        halfLst=(start+end)//2
        return max(largest2(lst,start, halfLst), largest2(lst,halfLst, end))