title: Why simply select a select syntax can save null?
body: <p>This is a question from Leetcode:
  <a href="https://i.stack.imgur.com/ed2Ak.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ed2Ak.png" alt="enter image description here" /></a></p>
  <p>I can't understand this approach, why it can overcome the issue?</p>
  <p><a href="https://i.stack.imgur.com/LdqOC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LdqOC.png" alt="
  " /></a></p>
  <p>I wonder the inside mechanism.</p>