Summary-Quick notes

Recursion

Definition: A method calls itself to solve smaller subproblems of the original problem.
Base Case: Stops the recursion; prevents infinite loops (e.g., when size = 1).
Recursive Call: Solves the smaller part of the problem (e.g., factorial(n-1)).
Call Stack: Each recursive call is added to the stack; unwinds once the base case is hit.
Key Example: Factorial → n! = n * factorial(n-1), with base case factorial(1) = 1.
Array Processing: Pass an index to process arrays recursively (e.g., sum elements).
Binary Search: Divide-and-conquer → Search left or right in a sorted array recursively.
Recursion vs Iteration: Recursion uses function calls; iteration uses loops.
Common Errors: Infinite recursion from missing or unreachable base cases.
Advantages: Simplifies problems like tree traversal or divide-and-conquer algorithms.

Methods and Control Structures

Basic Syntax + Structure: Focus on logic over syntax; readers will overlook small mistakes like missing semicolons or indentation. Know how to call/write methods, handle return values, and use parameters.
Arrays: Use for storing/accessing values. Syntax: int[] arr = new int[10];. Access elements with arr[index] and loop with for loops.
ArrayLists: Dynamic arrays with methods like add(), remove(), and size(). Syntax: ArrayList<Type> list = new ArrayList<Type>();.
Strings: Use .equals() for comparison (not ==). Use + for concatenation or StringBuilder for loops.
Control Flow: Master conditionals like if, else if, else.
Libraries: Assume all required classes and methods are pre-imported and simple to use (no complex APIs).

Classes

Class Header: Always declare the class as public unless the question specifies otherwise. Example: public class ClassNameHere.
Instance Variables: Must be private and initialized within the class. Example: private String message;. Avoid using static for instance variables unless explicitly stated.
Constructors:
- The constructor’s header must match the scoping of the class header (usually public).
- Initialize instance variables using this.variable = parameter;.
Methods:
- Declare methods as public unless otherwise noted.
- Use appropriate return types, such as void, int, or String.
Null Cases: Handle null or empty cases explicitly, as these often provide free points on FRQs. Example: Return null if a string is empty.
Formatting Tips: Always match the exact method and constructor names provided in the prompt to avoid losing points.
Number of Lines Example: Calculate using message.length() and divide by the width. Include a check for any remaining characters to add an extra line.
String Methods: Use .substring() and concatenation (+) for constructing or manipulating strings in methods like getLines().
Loops in Methods: Use for loops to iterate through strings or arrays based on problem requirements (e.g., constructing multi-line messages).
Inheritance: Use extends for inheritance and implements for interfaces if required. Ensure class headers include these keywords correctly.
Common Errors:
- Forgetting private for instance variables.
- Missing public in class or method headers.
- Incorrect initialization of instance variables.

Array/ArrayList

Understand the method signature (purpose, return type, access modifier).
Pay attention to input type (array vs. ArrayList).
Use loops with accurate bounds.
Handle null or empty cases.
Verify return type and syntax for arrays/ArrayLists before submitting.

2d Array

Remember there is some penalties for using some of the shortcuts ### Common Tasks:
- Sum all elements: Use a nested loop to accumulate values.
- Count specific values: Add conditions inside the loop.
- Find max/min: Compare current values during traversal.
### Null Cases:
- Check for null arrays or rows to avoid runtime errors.
### Array Dimensions:
- Use array.length for the number of rows and array[0].length for columns.
### Edge Cases:
- Handle edge cases like empty arrays, single-row arrays, or jagged arrays.