<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>

#include <bits/stdc++.h>
using namespace std;

class Solution {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        if (source == target)
            return 0;
        
        struct node{int bus; int lvl; node(int s, int l) : bus(s), lvl(l) {}};
        queue<node> q;

        unordered_map<int, unordered_set<int>> b2b;
        unordered_map<int, unordered_set<int>> s2b;
        int n = routes.size();

        //make stop to bus
        for (int i=0; i<n; i++){
            for (auto stop: routes[i]){
                s2b[stop].insert(i);
            }
        }

        //make bus to bus
        for (auto [stop, buses]: s2b)
            for (auto bus1: buses)
                for (auto bus2: buses)
                    b2b[bus1].insert(bus2);

        //find all bus source/target
        unordered_set<int> sourceBus;   
        unordered_set<int> sourceTarget;

        unordered_set<int> memo;
        for (auto v: s2b[source]){
            q.emplace(v, 1);
            // cout << "source: " << v << endl;
            memo.insert(v);
        }
        
        for (auto v: s2b[target]){
            sourceTarget.insert(v);
            // cout << "target: " << v << endl;
        }

        while (q.size()){
            auto [bus, lvl] = q.front();
            q.pop();
            // cout << "playing: " << bus << " " << lvl << endl;
            
            if (sourceTarget.find(bus) != sourceTarget.end()) return lvl;

            for (auto v: b2b[bus]){
                
                if (memo.find(v) != memo.end()) continue;
                memo.insert(v);
                q.emplace(v, lvl+1);
            }
        }

        return -1;
    
    }
};
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