src/main/java/com/fishercoder/solutions/_841.java

package com.fishercoder.solutions;

import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Set;

/**
 * 841. Keys and Rooms
 *
 * There are N rooms and you start in room 0.
 * Each room has a distinct number in 0, 1, 2, ..., N-1, and each room may have some keys to access the next room.
 * Formally, each room i has a list of keys rooms[i], and each key rooms[i][j] is an integer in [0, 1, ..., N-1] where N = rooms.length.
 * A key rooms[i][j] = v opens the room with number v.
 * Initially, all the rooms start locked (except for room 0).
 * You can walk back and forth between rooms freely.
 * Return true if and only if you can enter every room.
 *
 * Example 1:
 * Input: [[1],[2],[3],[]]
 * Output: true
 * Explanation:
 * We start in room 0, and pick up key 1.
 * We then go to room 1, and pick up key 2.
 * We then go to room 2, and pick up key 3.
 * We then go to room 3.  Since we were able to go to every room, we return true.
 *
 * Example 2:
 * Input: [[1,3],[3,0,1],[2],[0]]
 * Output: false
 * Explanation: We can't enter the room with number 2.
 *
 * Note:
 * 1 <= rooms.length <= 1000
 * 0 <= rooms[i].length <= 1000
 * The number of keys in all rooms combined is at most 3000.
 * */
public class _841 {
    public static class Solution1 {
        public boolean canVisitAllRooms(List<List<Integer>> rooms) {
            Set<Integer> unvisitedRooms = new HashSet<>();
            for (int i = 1; i < rooms.size(); i++) {
                unvisitedRooms.add(i);
            }
            List<Integer> keys = rooms.get(0);
            Queue<Integer> queue = new LinkedList<>();
            for (int key : keys) {
                queue.offer(key);
            }
            while (!queue.isEmpty()) {
                int size = queue.size();
                for (int i = 0; i < size; i++) {
                    int roomIndex = queue.poll();
                    unvisitedRooms.remove(roomIndex);
                    for (int j = 0; j < rooms.get(roomIndex).size(); j++) {
                        Integer nextRoom = rooms.get(roomIndex).get(j);
                        if (unvisitedRooms.contains(nextRoom) && !queue.contains(nextRoom)) {
                            queue.offer(nextRoom);
                        }
                    }
                }
            }
            return unvisitedRooms.isEmpty();
        }
    }
}