Bug fixes and improvements (#1380)

* preorder, inorder, postorder -> pre-order, in-order, post-order

* Bug fixes

* Bug fixes

* Update what_is_dsa.md

* Sync zh and zh-hant versions

* Sync zh and zh-hant versions.

* Update performance_evaluation.md and time_complexity.md

* Add @khoaxuantu to the landing page.

* Sync zh and zh-hant versions

* Add @ khoaxuantu to the landing page of zh-hant and en versions.

docs/index.html

@@ -322,6 +322,13 @@
                         <br><sub>JS, TS</sub>
                     </a>
                 </div>
+                <div class="profile-cell">
+                    <a href="https://github.com/khoaxuantu">
+                        <img class="profile-img" src="assets/avatar/avatar_khoaxuantu.jpg" alt="Reviewer: khoaxuantu" />
+                        <br><b>khoaxuantu</b>
+                        <br><sub>Ruby</sub>
+                    </a>
+                </div>
                 <div class="profile-cell">
                     <a href="https://github.com/krahets">
                         <img class="profile-img" src="assets/avatar/avatar_krahets.jpg" alt="Reviewer: krahets" />