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# Runtime: 254 ms (Top 43.79%) | Memory: 16.1 MB (Top 46.67%)
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class Solution:
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    def trap(self, a: List[int]) -> int:
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        l = 0
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        r = len(a) - 1
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        maxl = 0
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        maxr = 0
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        res = 0
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        while l <= r:
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            if a[l] <= a[r]:
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                if a[l] >= maxl:
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                    maxl = a[l]  # update maxl if a[l] is >=
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                else:
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                    res += maxl - a[l]  # adding captured water when maxl>a[l]
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                l += 1
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            else:
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                if a[r] >= maxr:
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                    maxr = a[r]
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                else:
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                    res += maxr - a[r]
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                r -= 1
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        return res