Data Structures Cheat Sheet

lists

arr = [1, 2, 3]
arr.append(4)        # [1, 2, 3, 4]
arr.pop()            # Removes last element -> 4
arr.insert(1, 9)     # [1, 9, 2, 3]
arr.remove(2)        # Removes first occurrence of 2
arr.sort()           # In-place sort
sorted_arr = sorted(arr)  # Returns new sorted list
arr.reverse()        # In-place reverse

Tuples (Immutable Lists)

t = (1, 2, 3)
x, y, z = t  # Tuple unpacking

Sets (Unordered, Unique Elements)

// empty: s = set()
s = {1, 2, 3}
s.add(4)
s.remove(2)
s.discard(5)  # Safer than remove – no error
s1 = {1, 2, 3}
s2 = {2, 3, 4}
s1 & s2  # Intersection: {2, 3}
s1 | s2  # Union: {1, 2, 3, 4}
s1 - s2  # Difference: {1}

Dictionaries

d = {'a': 1, 'b': 2}
d['c'] = 3           # Add key
value = d['a']       # Access value
value = d.get('x', 0)  # Safer access
del d['b']           # Remove key
for key, val in d.items():
    print(key, val)

Stack & Queues

stack = []
stack.append(1)
stack.append(2)
stack.pop()       # LIFO

from collections import deque
queue = deque()
queue.append(1)
queue.popleft()   # FIFO

Variable	Created As	Used For	Typical Methods
`list`	`[]`	general storage	`append()`, `insert()`, `pop(i)`, `sort()`
`stack`	`[]`	LIFO	`append()`, `pop()`
`queue`	`deque()`	FIFO	`append()`, `popleft()`

// join items in stack into one string
stack = []
joined = ''.join(stack)

Counter

from collections import Counter

nums = [1, 1, 2, 3, 2, 1]

c = Counter(nums)
print(c)

// Counter({1: 3, 2: 2, 3: 1})

word = "banana"
c = Counter(word)
print(c)

// Counter({'a': 3, 'n': 2, 'b': 1})

for key, value in Counter([1, 1, 2, 2, 2, 3]).items():
    print(f"{key} appears {value} times")

count[3]  # ➝ 3
count[99]  # ➝ 0
count.get(99, 0)  # ➝ 0

max_freq = max(count.values())  # ➝ 3
min_freq = min(count.values())  # ➝ 1

# One Most Frequent Element
most_common = count.most_common(1)[0]  # ➝ (3, 3)
elem = most_common[0]  # ➝ 3
freq = most_common[1]  # ➝ 3