https://leetcode.com/problems/course-schedule-ii/

courseScheduleii.js

/**
 * @param {number} numCourses
 * @param {number[][]} prerequisites
 * @return {number[]}
 */
var findOrder = function(numCourses, prerequisites) {
    
    const graph = new Array(numCourses).fill(null).map(() => []);
    
    prerequisites.forEach(([course, req]) => graph[course].push(req));
    
    const result = [];
    const backtrace = new Set();
    const seen = new Set();
    
    const dfs = (node) => { // returns true if cycle detected
        
        if(backtrace.has(node)) return true;
        if(seen.has(node)) return false;
        
        seen.add(node);
        backtrace.add(node);
        
        for(let i=0; i<graph[node].length; i++) if(dfs(graph[node][i])) return true;
        
        backtrace.delete(node);
        result.push(node);
        return false;
    };
    
    for(let i=0; i<numCourses; i++){
        if(dfs(i)) return [];
    }
    
    return result;
};