Data Structures and Algorithms in Java.
The easiest way to run an Applet in IntelliJ is by creating an Applet Run/Debug Configuration. Just click on the little dropdown that shows the name of your main class to execute, click on "Edit Configurations...", then click the + button, then choose "Applet" and specify your YourApplet as the target Applet class.
For succesed run user should dowload and install Java Applet Support plugin from Jetbrains: https://plugins.jetbrains.com/plugin/13148-java-applets-support/versions