If I have a list of characters, how would I return its most two frequencies with the less time complexity?

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Python

I'm using python language.
say the list = [a, b, a, e, g, a, b]
so, the output should be: a, b

What I have tried:

I have tried this algorithm:

1- make a dictionary and put the characters without repeating. #{a, b, e, g}
2- list = []
3- for each character in the dictionary:
if the character doesn't exist in the list:
count the character and append it to the list
4- sort the list, with reverse
5- print the first two characters

Will it be O(n)?
and if not, how will I make an algorithm that solve this problem with the possible less time complexity?