Describe the Painting LeetCode solution in Python

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class Solution:
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    def splitPainting(self, segments: List[List[int]]) -> List[List[int]]:
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        mix, res, last_i = DefaultDict(int), [], 0
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        for start, end, color in segments:
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            mix[start] += color
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            mix[end] -= color
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        for i in sorted(mix.keys()):
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            if last_i in mix and mix[last_i]:
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                res.append([last_i, i, mix[last_i]])
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                mix[i] += mix[last_i]
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            last_i = i
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        return res