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802. Find Eventual Safe States 👍

  • Time: $O(|V| + |E|)$
  • Space: $O(|V| + |E|)$
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enum class State { kInit, kVisiting, kVisited };

class Solution {
 public:
  vector<int> eventualSafeNodes(vector<vector<int>>& graph) {
    vector<int> ans;
    vector<State> state(graph.size());

    for (int i = 0; i < graph.size(); ++i)
      if (!hasCycle(graph, i, state))
        ans.push_back(i);

    return ans;
  }

 private:
  bool hasCycle(const vector<vector<int>>& graph, int u, vector<State>& state) {
    if (state[u] == State::kVisiting)
      return true;
    if (state[u] == State::kVisited)
      return false;

    state[u] = State::kVisiting;
    for (const int v : graph[u])
      if (hasCycle(graph, v, state))
        return true;
    state[u] = State::kVisited;

    return false;
  }
};

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enum State { kInit, kVisiting, kVisited }

class Solution {
  public List<Integer> eventualSafeNodes(int[][] graph) {
    List<Integer> ans = new ArrayList<>();
    State[] state = new State[graph.length];

    for (int i = 0; i < graph.length; ++i)
      if (!hasCycle(graph, i, state))
        ans.add(i);

    return ans;
  }

  private boolean hasCycle(int[][] graph, int u, State[] state) {
    if (state[u] == State.kVisiting)
      return true;
    if (state[u] == State.kVisited)
      return false;

    state[u] = State.kVisiting;
    for (final int v : graph[u])
      if (hasCycle(graph, v, state))
        return true;
    state[u] = State.kVisited;

    return false;
  }
}

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from enum import Enum


class State(Enum):
  kInit = 0
  kVisiting = 1
  kVisited = 2


class Solution:
  def eventualSafeNodes(self, graph: List[List[int]]) -> List[int]:
    state = [State.kInit] * len(graph)

    def hasCycle(u: int) -> bool:
      if state[u] == State.kVisiting:
        return True
      if state[u] == State.kVisited:
        return False

      state[u] = State.kVisiting
      if any(hasCycle(v) for v in graph[u]):
        return True
      state[u] = State.kVisited

    return [i for i in range(len(graph)) if not hasCycle(i)]