207-canFinish.js

function createGraph(numCourses, edges) {
    const graph = Array.from({ length: numCourses }, () => []);

    for (let edge of edges) {
        let [a, b] = edge;

        if (!(a in graph)) graph[a] = [];
        if (!(b in graph)) graph[b] = [];

        graph[a].push(b);
    }
    return graph;
}

function canFinish(numCourses, preq) {
    const graph = createGraph(numCourses, preq);
    let seen = new Set();
    let seeing = new Set();

    function explore(course) {
        if (seen.has(course)) return true;
        if (seeing.has(course)) return false;

        seeing.add(course);
        for (let neighbor of graph[course]) {
            if (!explore(neighbor)) return false;
        }

        seen.add(course);
        seeing.delete(course);
        return true;
    }

    for (let i = 0; i < numCourses; i++) {
        if (!explore(i)) return false;
    }
    return true;
}