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# Runtime: 81 ms (Top 17.97%) | Memory: 13.9 MB (Top 73.17%)
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class Solution:
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    def largeGroupPositions(self, s: str) -> List[List[int]]:
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        i = 0
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        c = 1
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        prev = ""
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        l = len(s)
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        ans = []
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        while i < l:
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            if s[i] == prev:
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                c += 1
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                if (i == l - 1) & (c >= 3):
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                    ans.append([i + 1 - c, i])
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            else:
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                if c >= 3:
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                    ans.append([i - c, i - 1])
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                c = 1
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            prev = s[i]
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            i += 1
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        return ans