1557.Minimum Number of Vertices to Reach All Nodes

tags: Medium,Graph

1557. Minimum Number of Vertices to Reach All Nodes

題目描述

Given a directed acyclic graph, with n vertices numbered from 0 to n-1, and an array edges where edges[i] = [

$fro{m}_i$,

$t{o}_i$] represents a directed edge from node

$fro{m}_i$ to node

$t{o}_i$.

Find the smallest set of vertices from which all nodes in the graph are reachable. It's guaranteed that a unique solution exists.

Notice that you can return the vertices in any order.

範例

Example 1:

Input: n = 6, edges = [[0,1],[0,2],[2,5],[3,4],[4,2]]
Output: [0,3]
Explanation: It's not possible to reach all the nodes from a single vertex. From 0 we can reach [0,1,2,5]. From 3 we can reach [3,4,2,5]. So we output [0,3].

Example 2:

Input: n = 5, edges = [[0,1],[2,1],[3,1],[1,4],[2,4]]
Output: [0,2,3]
Explanation: Notice that vertices 0, 3 and 2 are not reachable from any other node, so we must include them. Also any of these vertices can reach nodes 1 and 4.

Constraints:

• 2 <= n <= 10⁵
• 1 <= edges.length <= min(10⁵, n * (n - 1) / 2)
• edges[i].length == 2
• 0 <=
  $fro{m}_i$,
  $t{o}_i$ < n
• All pairs (
  $fro{m}_i$,
  $t{o}_i$) are distinct.

解答

Python

class Solution:
    def findSmallestSetOfVertices(self, n: int, edges: List[List[int]]) -> List[int]:
        return list(set(range(n)) - set(to for _, to in edges))

Yen-Chi ChenThu, May 18, 2023

Javascript

function findSmallestSetOfVertices(n, edges) {
  const set = new Set();
  for (const [from, to] of edges) {
    set.add(to);
  }

  const result = [];
  for (let i = 0; i < n; i++) {
    if (!set.has(i)) result.push(i);
  }

  return result;
}

這題直接用Set來記錄就行了，讓我想到1579題，當時天真的以為用Set就可以ㄏㄏ
MarsgoatThu, May 18, 2023

Reference

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