python中如何不用max和min函数求list中最大最小值？

python中如何不用max和min函数求list中最大最小值？并求差值

例如
[1,2,3]—2
[1,2,3,4]-3

def maxnum(lst):
    maxn = 0
    for i in lst:
        if maxn < i:
            maxn = i
    return maxn
def minnum(lst):
    minn = lst[0]
    for i in lst:
        if minn > i:
            minn= i
    return minn
a = [3,4,6,7]
print(maxnum(a))
print(minnum(a))