<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 {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        if (source == target) return 0;
        unordered_map<int, unordered_set<int>> stb;
        for (int i = 0; i < routes.size(); ++i) {
            for (int s : routes[i]) {
                stb[s].insert(i);
            }
        }
        vector<unordered_set<int>> adj(routes.size());
        for (int i = 0; i < routes.size(); ++i) {
            for (int s : routes[i]) {
                adj[i].insert(stb[s].begin(), stb[s].end());
            }
        }
        unordered_set<int> ts(stb[target].begin(), stb[target].end());
        vector<bool> visited(routes.size());
        vector<int> dp(routes.size(), -2);
        auto dfs = [&](auto &&self, int r) {
            if (dp[r] != -2) return dp[r];
            if (ts.contains(r)) return dp[r] = 1;
            visited[r] = true;
            int b = -1;
            for (int rr : adj[r]) {
                if (!visited[rr]) {
                    int a = self(self, rr);
                    if (a != -1 && (b == -1 || a + 1 < b)) b = a + 1;
                }
            }
            visited[r] = false;
            return dp[r] = b;
        };
        int b = -1;
        for (int r : stb[source]) {
            int a = dfs(dfs, r);
            if (a != -1 && (b == -1 || a < b)) b = a;
        }
        return b;
    }
};
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