combine two list and sort recursively

问题

I'm trying to define a function named combine to take two lists as parameters and combine them into one sorted list recursively.

def combine (l1,l2):
    if l1 == []:
        return l2
    elif l2 == []:
        return l1
    elif l1 == [] and l2 == []:
        return []
    sort_l = []
    index = 0
    for num in l1:
        if num <= l2[0]:
            sort_l.append(num)
            index += 1
        else:
            return combine(l2, l1[index:])
    return sort_l + l2

It gives me:

combine([1,3,5,7,9],[0,2,4,6,8]) -> [8, 9] 

but should:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Any advice???

回答1:

Your approach is correct. But, you are reallocating the sort_l in every recursive call. You need to either define it outside the function or pass it with every function call, after creating only once:

sort_l = []

def combine (l1,l2):
    if l1 == []:
        return l2
    elif l2 == []:
        return l1
    elif l1 == [] and l2 == []:
        return []
    index = 0
    for num in l1:
        if num <= l2[0]:
            sort_l.append(num)
            index += 1
        else:
            return combine(l2, l1[index:]) 
    return sort_l + l2

or

def combine (l1,l2, sort_l=None):
    if sort_l is None:
       sort_l = []
    if l1 == []:
        return l2
    elif l2 == []:
        return l1
    elif l1 == [] and l2 == []:
        return []
    index = 0
    for num in l1:
        if num <= l2[0]:
            sort_l.append(num)
            index += 1
        else:
            return combine(l2, l1[index:], sort_l) # pass it here
    return sort_l + l2

combine([1,3,5,7,9],[0,2,4,6,8]) # Don't pass any list, it will be created in first call.

回答2:

python lists can be combined with a simple + and the builtin function sorted() will sort items in the list for you.

sorted(l1 + l2) = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

回答3:

program should be like this:

sort_l = []
def combine (l1,l2):
    if l1 == []:
        return l2
    elif l2 == []:
        return l1
    elif l1 == [] and l2 == []:
        return []
    index = 0
    for num in l1:
        if num <= l2[0]:
            sort_l.append(num)
            index += 1
        else:
            return combine(l2, l1[index:])
    return sort_l + l2
print(combine([1,3,5,7,9],[0,2,4,6,8]))

回答4:

You're close. The trick is to put the smaller of l1[0] and l2[0] in front of the result of the recursive combine():

def combine (l1,l2):
    if l1 == []:
        return l2
    if l2 == []:
        return l1
    if l1[0] <= l2[0]:
        return [l1[0]] + combine(l1[1:], l2)
    return [l2[0]] + combine(l1, l2[1:])

来源：https://stackoverflow.com/questions/36990057/combine-two-list-and-sort-recursively