在不使用(集合)库的情况下查找双字母数

def find_double_letter():
    double_letters = []
    for i in range(26):
        for j in range(26):
            letter1 = chr(ord('a') + i)
            letter2 = chr(ord('a') + j)
            double_letters.append(letter1 + letter2)
            double_letters.append(letter1.upper() + letter2)
    return double_letters

result = find_double_letter()
print(result)