1. Reverse a string in place. 2. Write a method that converts a decimal number to binary. 3. What is a Singleton? 4. What are the differences between Array and ArrayList in Java? 5. If you were to design a phone directory, what data structure would you use? 6. Calculate Fibonacci numbers with and without recursion; for example, given an array [3, 5, 4], return the 3rd, 5th, and 4th Fibonacci numbers in an array. What is the complexity of your algorithm? 7. Return the length of the last word in a sentence. For example: String s = "Hello Wang" should return 4; String s2 = "Hello " should return 5; String s3 = "" should return 0. 8. Given a number greater than 9, write a program that repeatedly sums its digits until the result is less than 10 (e.g., 38 → 3+8=11, then 1+1=2, stop). 9. What are the differences between String, StringBuilder, and StringBuffer in Java?