l = [1,2,3,4,5]
m = [1,3,4,5,10,8]

commmon_elements = []


def a():
    for li in l:
        for mi in m:
            if mi == li:
                commmon_elements.append(li)
    
def b():
    lm = l + m
    
    counter = dict()
    for el in  lm:
    
        if el not in counter:
            counter[el] = 0
        
        counter[el] += 1 
    
    
    for key, count in counter.items():
        if count > 1:
            commmon_elements.append(key)


def c():
    from collections import defaultdict

    counter = defaultdict(int)

    for el in l + m:
        counter[el] += 1

    commmon_elements.extend([key for key, value in counter.items() if value > 1])


def d():
    from collections import Counter

    counter = Counter(l + m)

    commmon_elements.extend(
        [key for key, value in counter.items() if value > 1]
    )


d()

print("result: ", commmon_elements)
print("correct answer: [1, 3, 4, 5]")