Hi, I'm trying to find to longest sequence of the same letters between two words. In the example below I want to return "nna" as the longest sequence

Question

def longest_common_substring(string1, string2):
    list = []
    for letter1 in string1:
        for letter2 in string2:
            if letter1 == letter2:
                list.append(letter2)
    return list
print(longest_common_substring("anna", "nna"))

>> ['a', 'n', 'n', 'n', 'n', 'a']