<p>You are given an array <code>routes</code> representing bus routes where <code>routes[i]</code> is a bus route that the <code>i<sup>th</sup></code> bus repeats forever.</p>

<ul>
	<li>For example, if <code>routes[0] = [1, 5, 7]</code>, this means that the <code>0<sup>th</sup></code> bus travels in the sequence <code>1 -&gt; 5 -&gt; 7 -&gt; 1 -&gt; 5 -&gt; 7 -&gt; 1 -&gt; ...</code> forever.</li>
</ul>

<p>You will start at the bus stop <code>source</code> (You are not on any bus initially), and you want to go to the bus stop <code>target</code>. You can travel between bus stops by buses only.</p>

<p>Return <em>the least number of buses you must take to travel from </em><code>source</code><em> to </em><code>target</code>. Return <code>-1</code> if it is not possible.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> routes = [[1,2,7],[3,6,7]], source = 1, target = 6
<strong>Output:</strong> 2
<strong>Explanation:</strong> The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> routes = [[7,12],[4,5,15],[6],[15,19],[9,12,13]], source = 15, target = 12
<strong>Output:</strong> -1
</pre>

<p>&nbsp;</p>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= routes.length &lt;= 500</code>.</li>
	<li><code>1 &lt;= routes[i].length &lt;= 10<sup>5</sup></code></li>
	<li>All the values of <code>routes[i]</code> are <strong>unique</strong>.</li>
	<li><code>sum(routes[i].length) &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= routes[i][j] &lt; 10<sup>6</sup></code></li>
	<li><code>0 &lt;= source, target &lt; 10<sup>6</sup></code></li>
</ul>

class Solution:
    def numBusesToDestination(self, routes: List[List[int]], source: int, target: int) -> int:
        # stop : routes
        stops_routes = {}
        # routes : stop
        bus_routes = {}

        if source == target:
            return 0

        for i, route in enumerate(routes):
            for stop in route:
                stops_routes[stop] = stops_routes.get(stop, []) + [i]
            
            bus_routes[i] = route

        # print(stops_routes)
        # print(bus_routes)
        bus_routes_visited = set()
        queue = deque()
        # stop, buses_taken, currr_bus
        queue.append((source,0,-1))

        # print(stops_routes)

        while queue:
            # print(queue)
            extract = queue.popleft()
            curr_stop = extract[0]
            curr_buses_taken = extract[1]
            current_bus = extract[2]

            for bus_routes_available in stops_routes.get(curr_stop, []):
                if bus_routes_available not in bus_routes_visited:
                    bus_routes_visited.add(bus_routes_available)
                    for stop in bus_routes.get(bus_routes_available,[]):
                        if stop == target:
                            return curr_buses_taken + 1
                        else:
                            queue.append((stop, curr_buses_taken + 1, bus_routes_available))

        return -1




                        






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