N-Queens LeetCode solution in Python

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class Solution:
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    def solveNQueens(self, n: int) -> List[List[str]]:
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        coord = self.findNextRows(0, n)
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        ans = []
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        for c in coord:
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            temp = []
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            for j in c:
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                temp.append("." * j + "Q" + "." * (n - j - 1))
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            ans.append(temp)
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        return ans
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    def findNextRows(self, i, n, h_occ=set(), d_occ=set(), ad_occ=set()):
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        """
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        h_occ: occupied horizontal coordinate
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        d_occ: occupied diagonal
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        ad_occ: occupied anti-diagonal
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        """
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        ans = []
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        if i == n:
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            return [[]]
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        for j in range(n):
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            if (
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                (j not in h_occ)
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                and (j - i not in d_occ)
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                and ((j - n + 1) + i not in ad_occ)
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            ):
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                h_occ.add(j)
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                d_occ.add(j - i)
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                ad_occ.add((j - n + 1) + i)
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                temp = self.findNextRows(i + 1, n, h_occ, d_occ, ad_occ)
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                h_occ.remove(j)
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                d_occ.remove(j - i)
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                ad_occ.remove((j - n + 1) + i)
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                ans += [[j] + l for l in temp]
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        return ans