Technical Questions

• Why all my solutions in the code editor suddenly went missing?
• I encountered Wrong Answer/Runtime Error for a specific test case. When I test my code using this test case, it produced the correct output. Why?
• What does [1,null,2,3] mean in binary tree representation?
• Where do I find all of my submitted code?
• What are the environments for the programming languages?
• Am I allowed to print something to stdout?