Skip to content

802. Find Eventual Safe States 👍

  • Time: $O(|V| + |E|)$
  • Space: $O(|V| + |E|)$
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
enum class State { kInit, kVisiting, kVisited };

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

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

    return ans;
  }

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

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

    return false;
  }
};

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
enum State { kInit, kVisiting, kVisited }

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

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

    return ans;
  }

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

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

    return false;
  }
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
from enum import Enum


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


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

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

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

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