topological-sorting

// https://leetcode.com/explore/learn/card/graph/623/kahns-algorithm-for-topological-sorting/3868/
/**
 * @param {number} numCourses
 * @param {number[][]} prerequisites
 * @return {number[]}
 */
var findOrder = function(n, classes) {
    // There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You  are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai.

    const adj = [...Array(n)].map(()=>new Set());
    for(const [a, b] of classes) {
        adj[a].add(b);  //  indegree // incoming
    }
    const que = [];
    for(let i=0; i<adj.length; i++) {
        let set = adj[i];
        if(set.size == 0) {
            que.push(i);
        }
    }
    if(!que.length) return [];
    let ans = [];
    while(que.length) {
        const node = que.shift();
        ans.push(node);
        for(let i=0; i<adj.length; i++) {
            const set = adj[i];
            if(set.has(node)) {
                set.delete(node);
                if(set.size == 0) {
                    que.push(i);
                }
            }
        }
    }
    for(let i=0; i<adj.length; i++) {
        if(adj[i].size > 0) return [];
    }
    return ans;
};