841. Keys and Rooms

Problem

There are n rooms labeled from 0 to n - 1 and all the rooms are locked except for room 0. Your goal is to visit all the rooms. However, you cannot enter a locked room without having its key.

When you visit a room, you may find a set of distinct keys in it. Each key has a number on it, denoting which room it unlocks, and you can take all of them with you to unlock the other rooms.

Given an array rooms where rooms[i] is the set of keys that you can obtain if you visited room i, return true **if you can visit *all* the rooms, or** false otherwise.

  Example 1:

Input: rooms = [[1],[2],[3],[]]
Output: true
Explanation: 
We visit room 0 and pick up key 1.
We then visit room 1 and pick up key 2.
We then visit room 2 and pick up key 3.
We then visit room 3.
Since we were able to visit every room, we return true.

Example 2:

Input: rooms = [[1,3],[3,0,1],[2],[0]]
Output: false
Explanation: We can not enter room number 2 since the only key that unlocks it is in that room.

  Constraints:

• n == rooms.length

• 2 <= n <= 1000

• 0 <= rooms[i].length <= 1000

• 1 <= sum(rooms[i].length) <= 3000

• 0 <= rooms[i][j] < n

• All the values of rooms[i] are unique.

Solution (Java)

class Solution {
    public boolean canVisitAllRooms(List<List<Integer>> rooms) {
        Set<Integer> visited = new HashSet<>();
        visited.add(0);
        TreeSet<Integer> treeSet = new TreeSet<>(rooms.get(0));
        while (!treeSet.isEmpty()) {
            Integer key = treeSet.pollFirst();
            if (!visited.add(key)) {
                continue;
            }
            if (visited.size() == rooms.size()) {
                return true;
            }
            treeSet.addAll(rooms.get(key));
        }
        return visited.size() == rooms.size();
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).