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1557. Minimum Number of Vertices to Reach All Nodes 👍

  • Time: $O(|E|)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<int> findSmallestSetOfVertices(int n, vector<vector<int>>& edges) {
    vector<int> ans;
    vector<int> inDegrees(n);

    for (const vector<int>& edge : edges)
      ++inDegrees[edge[1]];

    for (int i = 0; i < inDegrees.size(); ++i)
      if (inDegrees[i] == 0)
        ans.push_back(i);

    return ans;
  }
};

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class Solution {
  public List<Integer> findSmallestSetOfVertices(int n, List<List<Integer>> edges) {
    List<Integer> ans = new ArrayList<>();
    int[] inDegrees = new int[n];

    for (List<Integer> edge : edges)
      ++inDegrees[edge.get(1)];

    for (int i = 0; i < inDegrees.length; ++i)
      if (inDegrees[i] == 0)
        ans.add(i);

    return ans;
  }
}

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class Solution:
  def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
    inDegrees = [0] * n

    for _, v in edges:
      inDegrees[v] += 1

    return [i for i, d in enumerate(inDegrees) if d == 0]