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+<h2><a href="https://leetcode.com/problems/keys-and-rooms/?envType=study-plan-v2&envId=leetcode-75">871. Keys and Rooms</a></h2><h3>Medium</h3><hr><p>There are <code>n</code> rooms labeled from <code>0</code> to <code>n - 1</code>&nbsp;and all the rooms are locked except for room <code>0</code>. Your goal is to visit all the rooms. However, you cannot enter a locked room without having its key.</p>
+
+<p>When you visit a room, you may find a set of <strong>distinct keys</strong> 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.</p>
+
+<p>Given an array <code>rooms</code> where <code>rooms[i]</code> is the set of keys that you can obtain if you visited room <code>i</code>, return <code>true</code> <em>if you can visit <strong>all</strong> the rooms, or</em> <code>false</code> <em>otherwise</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> rooms = [[1],[2],[3],[]]
+<strong>Output:</strong> true
+<strong>Explanation:</strong> 
+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.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> rooms = [[1,3],[3,0,1],[2],[0]]
+<strong>Output:</strong> false
+<strong>Explanation:</strong> We can not enter room number 2 since the only key that unlocks it is in that room.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>n == rooms.length</code></li>
+	<li><code>2 &lt;= n &lt;= 1000</code></li>
+	<li><code>0 &lt;= rooms[i].length &lt;= 1000</code></li>
+	<li><code>1 &lt;= sum(rooms[i].length) &lt;= 3000</code></li>
+	<li><code>0 &lt;= rooms[i][j] &lt; n</code></li>
+	<li>All the values of <code>rooms[i]</code> are <strong>unique</strong>.</li>
+</ul>