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⟨/⟩ Computer Science
A-Level CS Revision Hub
Components of a Computer
CPU, Memory, Storage, I/O. The Von Neumann architecture describes how instructions and data share the same memory and bus.
Networking
OSI model, TCP/IP, HTTP/HTTPS, DNS, routers, switches. LAN vs WAN. Client-server vs peer-to-peer models.
Cyber Security
Social engineering, malware, phishing, encryption (symmetric/asymmetric), hashing, firewalls, penetration testing.
Databases
Relational databases, SQL, primary/foreign keys, normalisation (1NF, 2NF, 3NF), ACID properties, transactions.
Boolean Algebra
Logic gates, truth tables, Karnaugh maps, De Morgan's laws. Simplification of boolean expressions.
Operating Systems
Process scheduling (FCFS, SJF, Round Robin), memory management, virtual memory, file systems, interrupts.
Binary Search — O(log n)
def binary_search(arr, target):
low, high = 0, len(arr) - 1
while low <= high:
mid = (low + high) // 2
if arr[mid] == target:
return mid
elif arr[mid] < target:
low = mid + 1
else:
high = mid - 1
return -1Bubble Sort — O(n²)
def bubble_sort(arr):
n = len(arr)
for i in range(n):
for j in range(0, n-i-1):
if arr[j] > arr[j+1]:
arr[j], arr[j+1] = arr[j+1], arr[j]
return arrMerge Sort — O(n log n)
def merge_sort(arr):
if len(arr) <= 1:
return arr
mid = len(arr) // 2
left = merge_sort(arr[:mid])
right = merge_sort(arr[mid:])
return merge(left, right)Big O Complexity
OOP Principles
- Encapsulation — bundling data and methods
- Inheritance — subclasses inherit from superclasses
- Polymorphism — same interface, different behaviour
- Abstraction — hiding implementation details
Recursion
A function that calls itself. Must have a base case to stop recursion. Uses the call stack.
def factorial(n):
if n == 0: # base case
return 1
return n * factorial(n-1)Data Structures
- Stack — LIFO (Last In First Out)
- Queue — FIFO (First In First Out)
- Linked List — nodes with pointers
- Tree — hierarchical, root/leaf nodes
- Hash Table — key-value, O(1) lookup
- Graph — vertices and edges
Programming Paradigms
- Imperative — step-by-step instructions
- OOP — objects and classes
- Functional — pure functions, no side effects
- Declarative — what, not how (SQL, HTML)
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