Longest Cycle in a Graph LeetCode solution in Python

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class Solution:
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    def longestCycle(self, edges: List[int]) -> int:
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        preorder = [-1 for _ in range(len(edges))]
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        self.ans = -1
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        self.pre = 0
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        def dfs(self, i: int, st: int) -> None:
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            preorder[i] = self.pre
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            self.pre += 1
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            if edges[i] == -1:
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                return
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            elif preorder[edges[i]] == -1:
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                dfs(self, edges[i], st)
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                return
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            elif preorder[edges[i]] >= st:
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                self.ans = max(self.ans, preorder[i] - preorder[edges[i]] + 1)
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                return
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        for i in range(len(edges)):
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            if preorder[i] == -1 and edges[i] != -1:
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                dfs(self, i, self.pre)
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        return self.ans