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.
For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1.
Return true if you can finish all courses. Otherwise, return false.
Example 1:
Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take.
To take course 1 you should have finished course 0. So it is possible.
Example 2:
Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take.
To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.
Constraints:
1 <= numCourses <= 2000
0 <= prerequisites.length <= 5000
prerequisites[i].length == 2
0 <= ai, bi < numCourses
All the pairs prerequisites[i] are unique.
/**
* @param {number} numCourses
* @param {number[][]} prerequisites
* @return {boolean}
*/
var canFinish = function(numCourses, prerequisites) {
const incommingEdges = new Array(numCourses).fill(0);
const adjs = {};
// create adjacent listx
// couting num of edges incomming
for (const [here, there] of prerequisites) {
if (adjs[here] === undefined) adjs[here] = [];
adjs[here].push(there);
incommingEdges[there] += 1;
}
const q = [];
// find vertexes incomming edges 0
for (let i = 0; i < incommingEdges.length; i++) {
if (incommingEdges[i] === 0) {
q.push(i);
}
}
// iterate queue until there is no vertex, whose the number of incomming edges is equal to zero
const seq = [];
while (q.length > 0) {
const here = q.shift();
seq.push(here);
const adj = adjs[here] ?? [];
// if (adj === undefined) continue;
for (const there of adj) {
incommingEdges[there] -= 1;
if (incommingEdges[there] === 0) {
q.push(there);
}
}
}
return seq.length === numCourses ? true : false;
};
