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# Runtime: 450 ms (Top 49.65%) | Memory: 14.6 MB (Top 82.17%)
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class Solution:
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    def balancedString(self, s):
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        count = collections.Counter(s)
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        res = n = len(s)
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        if all(n / 4 == count[char] for char in "QWER"):
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            return 0
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        left = 0
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        for right, char in enumerate(s):
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            # replace char whose index==right to check if it is balanced
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            count[char] -= 1
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            # if it is balanced, window shrinks to get the smallest length of window
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            while left <= right and all(n / 4 >= count[char] for char in "QWER"):
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                res = min(res, right - left + 1)
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                count[s[left]] = count[s[left]] + 1
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                left += 1
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        return res