87.Keys_and_Rooms.py

# 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.

class Solution:
    def canVisitAllRooms(self, rooms: list) -> bool:
        seen = [False] * len(rooms)
        seen[0] = True
        stack = [0]
        #At the beginning, we have a todo list "stack" of keys to use.
        #'seen' represents at some point we have entered this room.
        
        while stack:  #While we have keys...
            node = stack.pop() # get the next key 'node'
            for nei in rooms[node]: # For every key in room # 'node'...
                if not seen[nei]: # ... that hasn't been used yet
                    seen[nei] = True # mark that we've entered the room
                    stack.append(nei) # add the key to the todo list
                    
        return all(seen) # Return true iff we've visited every room