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int longestCycle(vector<int>& edges) {
        vector<pair<int, int>> took(edges.size()); // did we take the outgoing edge from i?
        int ans = -1;
        for (int node = 0; node < edges.size(); ++node) {
            int len = 1, cur = node;
            while (cur != -1 && took[cur].second == 0) {
                took[cur] = make_pair(node, len++);
                cur = edges[cur];
            } 
            if (cur != -1 && took[cur].first == node) {
                ans = max(ans, len - took[cur].second);
            }
        }
        return ans;
    }