0210-course-schedule-ii.ts

function findOrder(numCourses: number, prerequisites: number[][]): number[] {
    const prereq = [];
    for (let i = 0; i < numCourses; i++) {
        prereq[i] = [];
    }
    for (const [crs, pre] of prerequisites) {
        prereq[crs].push(pre);
    }

    const output = [];
    const visit = new Set();
    const cycle = new Set();

    function dfs(course: number): boolean {
        if (cycle.has(course)) {
            return false;
        }
        if (visit.has(course)) {
            return true;
        }

        cycle.add(course);
        for (const pre of prereq[course]) {
            if (!dfs(pre)) {
                return false;
            }
        }
        cycle.delete(course);
        visit.add(course);
        output.push(course);
        return true;
    }

    for (let j = 0; j < numCourses; j++) {
        if (!dfs(j)) {
            return [];
        }
    }

    return output;
}