Python：如何订购套装

Question

So ive been doing this assignment that returns the integer that doesn't have a matching integer or -(integer). so far I have:

def only_once(a):
    c = a
    z = []
    freq = Counter(c)
    for key, value in freq.items():
        if value == 1:
            z.append(key)
    return z

The main problem however, is the ordering. if I put in a large list, and then using the function it will just rearrange the list into a set order. So how do i make the ordering of z be like a but with the removed variables.

Answer 1

def only_once(a):
    c = a
    z = []
    freq = Counter(c)
    for key in a:
        if freq[key] == 1:
            z.append(key)
    return z

Or as a list comprehension

def only_once(a):
    freq = Counter(a)
    z = [key for key in a if freq[key] == 1]