package com.hit.basmath.interview.top_interview_questions.hard_collection.trees_and_graphs;

import java.util.LinkedList;
import java.util.Queue;

/**
 * 207. Course Schedule
 * <p>
 * There are a total of n courses you have to take, labeled from 0 to n-1.
 * <p>
 * Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]
 * <p>
 * Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?
 * <p>
 * Example 1:
 * <p>
 * Input: 2, [[1,0]]
 * Output: true
 * <p>
 * Explanation: There are a total of 2 courses to take.
 * To take course 1 you should have finished course 0. So it is possible.
 * <p>
 * Example 2:
 * <p>
 * Input: 2, [[1,0],[0,1]]
 * Output: false
 * <p>
 * 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.
 * <p>
 * Note:
 * <p>
 * 1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
 * 2. You may assume that there are no duplicate edges in the input prerequisites.
 */
public class _207 {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[][] matrix = new int[numCourses][numCourses]; // i -> j
        int[] indegree = new int[numCourses];

        for (int[] prerequisite : prerequisites) {
            int ready = prerequisite[0];
            int pre = prerequisite[1];
            if (matrix[pre][ready] == 0)
                indegree[ready]++; //duplicate case
            matrix[pre][ready] = 1;
        }

        int count = 0;
        Queue<Integer> queue = new LinkedList();
        for (int i = 0; i < indegree.length; i++) {
            if (indegree[i] == 0) queue.offer(i);
        }
        while (!queue.isEmpty()) {
            int course = queue.poll();
            count++;
            for (int i = 0; i < numCourses; i++) {
                if (matrix[course][i] != 0) {
                    if (--indegree[i] == 0)
                        queue.offer(i);
                }
            }
        }
        return count == numCourses;
    }
}