How to efficiently calculate prefix sum of frequencies of characters in a string?

Say, I have a string

s = 'AAABBBCAB'

How can I efficiently calculate the prefix sum of frequencies of each character in the string, i.e.:

psum = [{'A': 1}, {'A': 2}, {'A': 3}, {'A': 3, 'B': 1}, {'A': 3, 'B': 2}, {'A': 3, 'B': 3}, {'A': 3, 'B': 3, 'C': 1}, {'A': 4, 'B': 3, 'C': 1}, {'A': 4, 'B': 4, 'C': 1}]

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