Minimum Deletions to Make String Balanced LeetCode solution in Python

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class Solution:
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    def minimumDeletions(self, s: str) -> int:
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        preSum = [0] * (len(s) + 1)
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        sufSum = [0] * (len(s) + 1)
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        for i in range(len(s)):
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            if s[i] == "a":
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                preSum[i] += 1 + preSum[i - 1]
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            else:
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                preSum[i] = preSum[i - 1]
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            if s[len(s) - i - 1] == "b":
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                sufSum[len(s) - i - 1] += 1 + sufSum[len(s) - i]
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            else:
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                sufSum[len(s) - i - 1] += sufSum[len(s) - i]
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        maxStringLength = 0
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        for i in range(len(s)):
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            if preSum[i] + sufSum[i] > maxStringLength:
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                maxStringLength = preSum[i] + sufSum[i]
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        return len(s) - maxStringLength