1. Arrays / Lists

When Used:

Most problems begin with arrays
Often solved with two pointers, sliding window, or prefix sums

Key Operations:

Operation	Time
Access `arr[i]`	O(1)
Append	O(1)
Insert/delete middle	O(n)
Search	O(n)
Sort	O(n log n)

Useful Syntax:

nums = [1,2,3]

nums.append(4)
nums.pop()
nums.sort()
nums.reverse()

for i, v in enumerate(nums):
    pass

Patterns:

Two Pointers
Sliding Window
Prefix Sums
Kadane's Algorithm

2. Strings

Strings behave like arrays but are immutable.

Useful Operations:

s = "hello"

s[0]
s[1:4]
len(s)

s.lower()
s.upper()

s.split()
" ".join(list_of_words)

s.replace("a","b")

c.isdigit()
c.isalpha()

Common Problems:

Longest substring without repeating