title: Declaring a 2D vector
body: <p>In some cases only the below line works.Why so? </p>
  
  <pre><code>vector&lt; vector&lt;int&gt;&gt; a(M,N);
  </code></pre>
  
  <p>This works in every case.</p>
  
  <pre><code>vector&lt; vector&lt;int&gt;&gt; a(M, vector&lt;int&gt; (N));
  </code></pre>
  
  <p>What's the difference?</p>