递归函数在python上找到最大值

Question

I am stuck my code. I want to find recursive max value. here is my goal

1.If more than one element maximises the key, then the first one (the one that comes earliest in the array) must be returned.

2.The key parameter must be optional; if not provided, the function must return the (first) largest element. Think of a good default value for the key function!

3.Do not use the built-in max or min functions (obviously).

here is my code!

def recursive_max(seq, key):
    if len(seq) == 1:
        return seq[0]
    else:
        key = recursive_max(seq[1:])
        if key > seq[0]:
            return key
        else:
            return seq[0]
print(recursive_max(range(-5, 5 + 1))) #answer is 5
print(recursive_max(range(-5, 5 + 1), lambda x: x * x)) #answer is -5
class PoliticalDivision:
    def __init__(self, name, area):
        self.name = name
        self.area = area

divisions = [
    PoliticalDivision("Brazil", 8.5),
    PoliticalDivision("China", 9.5),
    PoliticalDivision("New Zealand", 0.27),
    PoliticalDivision("Russia", 17),
    PoliticalDivision("UK", 0.24),
    PoliticalDivision("US", 9.5),
]

print(recursive_max(divisions, lambda division: division.area).name) #answer is Russia.

I just cant get ritht output.

even another code is

def recursive_max(seq, key=lambda x: x):
    if len(seq) == 1:
        return seq[0]
    else:
        return max(seq[0], recursive_max(seq[1:], key), key=key)

the feedback is Runtime error

File "prog.python3", line 5, in recursive_max return max(seq[0], recursive_max(seq[1:], key), key=key)

how to improve it ? any suggestions will be glad :)

Answer 1

Consider:

def recursive_max(seq, key=None):
    # if key isn't given, call it again with key being returning the value itself
    if not key: return recursive_max(seq, lambda a: a)

    # error checking: can't get max of empty sequence
    if not seq: raise ValueError("max of empty seq")

    # base case: seq of 1, the max is the first element
    if len(seq) == 1: return seq[0]

    # get the max of the rest of the list
    sub_max = recursive_max(seq[1:], key)

    # if that's bigger than 1st element, return that, else return 1st element
    return sub_max if key(sub_max) > key(seq[0]) else seq[0]