title: How can I get pit in concave polygon, I have a polygon arr
body: <pre><code>let arr=[[4,3],[4,9],[7,8],[9,8],[10,10],[10,4]]
  </code></pre>
  
  <p>I know <code>[7,8],[9,8]</code> there it is pit, I want to write a function to get the concave point in this array how can I write.</p>
  
  <p>I've used graphics algorithms, but not for complex concave edges. Then I thought about whether I could handle the data that the server gave me, which is a polygon.</p>