Skip to content

841. Keys and Rooms

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:

class Solution {
    public boolean canVisitAllRooms(List<List<Integer>> rooms) {
      // room = node
      // key -> branch
      int n = rooms.size();
      Set<Integer> visited = new HashSet<>();
      dfs(0, rooms, visited);


      return visited.size() == n;
    }

    private void dfs(int i, List<List<Integer>> rooms, Set<Integer> visited){
        if (visited.contains(i)){
            return;
        }

        visited.add(i);
        List<Integer> next = rooms.get(i);
        for (int nei : next){
            dfs(nei, rooms, visited);
        }

        return;
    }
}
// TC: O(V+E)
// SC: O(V)