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1def longest_common_domain(email1, email2):
2    a = email1.split('@')[1]
3    b = email2.split('@')[1]
4    # Compare strings at the C speed
5    if a == b:
6        return a
7    min_len = min(len(a), len(b))
8    i = -1
9    while -i <= min_len and a[i] == b[i]:
10        i -= 1
11    return a[i:][1:]
12
13
14test1 = 'user01@dev.connex.io', 'admin@connex.io'
15test2 = 'testtest@gmail.com', 'foobar@mail.ru'
16test3 = 'testtest@gmail.com', 'foobar@gmail.com'
17test4 = 'testtest@4mail.com', 'foobar@5mail.com'
18
19result = longest_common_domain(*test1)
20print 'result:', result
21assert result == 'connex.io'
22
23result = longest_common_domain(*test2)
24print 'result:', result
25assert result == ''
26
27result = longest_common_domain(*test3)
28print 'result:', result
29assert result == 'gmail.com'
30
31result = longest_common_domain(*test4)
32print 'result:', result
33assert result == 'mail.com'