1557. Minimum Number of Vertices to Reach All Nodes

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    Problem

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

    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.

      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:

    Solution

    /**
     * @param {number} n
     * @param {number[][]} edges
     * @return {number[]}
     */
    var findSmallestSetOfVertices = function(n, edges) {
        var map = Array(n).fill(0);
        for (var i = 0; i < edges.length; i++) {
            map[edges[i][1]]++;
        }
        var res = [];
        for (var j = 0; j < n; j++) {
            if (map[j] === 0) {
                res.push(j);
            }
        }
        return res;
    };
    

    Explain:

    Because it's guaranteed that a unique solution exists, so we can simply collect nodes with no incoming edge.

    Complexity: