## 1. 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.

## 2. Solution

I solved this problem like this.

If we use stack, you can solve easily.

```
class Solution:
def canVisitAllRooms(self, rooms: List[List[int]]) -> bool:
visited = [0] * len(rooms)
visited[0] = 1
stack = rooms[0]
while stack:
next_rooms = stack.pop()
if visited[next_rooms]:
continue
visited[next_rooms] = 1
stack.extend(rooms[next_rooms])
return sum(visited) == len(rooms)
```