1. Python for DSA

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1️⃣ Basics & Syntax

📌 Dynamic Typing

n = 0
n = "abc"   # valid

• No type declaration
• Type decided at runtime

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📌 Multiple Assignment

a, b = 1, "hello"

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📌 Increment

n += 1
# ❌ n++ not allowed

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📌 Null Value

x = None

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📌 Conditionals

if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

• No parentheses
• Use indentation
• Use : after condition

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📌 Logical Operators

if a > 0 and b > 0:

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2️⃣ Loops

📌 While Loop

i = 0
while i < 5:
    print(i)
    i += 1

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📌 For Loop

for i in range(5):        # 0 → 4
for i in range(2, 6):     # 2 → 5
for i in range(5, 1, -1): # reverse 5 → 2

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3️⃣ Numbers & Math

📌 Division

5 / 2   # 2.5
5 // 2  # 2 (floor)

⚠️ Negative case:

-3 // 2  # -2 (floor)
# most language round down to -1
# workaround
int(-3/2) # -1 (floor)

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📌 Modulo

10 % 3   # 1
-10 % 3  # 2 (important!)
math.fmod(-10,3)

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📌 Useful Functions

import math
 
math.floor(x)
math.ceil(x)
math.sqrt(x)
pow(2, 3)
 
float("inf")
float("-inf")

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4️⃣ Lists (Arrays) - Stack

📌 Basics

nums = [1, 2, 3]
nums.append(4) # O(1)
nums.pop() # O(1)

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📌 Insert

nums.insert(1, 10)  # O(n)

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📌 Access

nums[0]
nums[-1]   # last element

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📌 Initialize

arr = [1] * 5

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📌 Slicing

nums[1:3]   # index 1 → 2

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📌 unpacking

a,b,c = [1,2,3]

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📌 Looping

for num in nums:
for i in range(len(nums)):
for i, val in enumerate(nums):

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📌 Reverse & Sort

nums.reverse()
nums.sort()
nums.sort(reverse=True)

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📌 Custom Sort

strs.sort(key=lambda x: len(x)) # by len of the string

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📌 List Comprehension

[expression for item in iterable if condition]
squares = [x * x for x in nums]
even_numbers = [x for x in range(10) if x % 2 == 0]
 
matrix = [[1, 2], [3, 4], [5, 6]]
flat = [num for row in matrix for num in row] # [1, 2, 3, 4, 5, 6]

Pattern	Meaning
[x for x in arr]	loop
[x for x in arr if cond]	filter
[f(x) for x in arr]	transform
[f(x) for x in arr if cond]	filter + transform
[x for a in A for x in a]	nested loop
[x if cond else y for x in arr]	if-else
[num for row in matrix for num in row]	flat matrix

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⚠️ 2D Array Pitfall

grid = [[0]*4 for _ in range(4)]  # ✅ correct
 
grid = [[0]*4]*4  # ❌ same row reference
grid[0][0] = 1 # [[1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0]]

Case	Use
1D list	[0]*n ✅
2D list	[[0]*n for i in range(m)] ✅

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5️⃣ Strings

📌 Basics

s = "hello"

• Immutable ❗

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📌 Operations

s + "world"
int("123")
str(123)

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📌 ASCII

ord('a')  # 97

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📌 Join

"".join(["a", "b", "c"]) # abc

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6️⃣ Queue (Deque)

from collections import deque
 
q = deque()
q.append(1) # deque([1])
q.appendleft(2) # deque([2,1])
q.pop() # 2 deque([1])
q.popleft() # 2 deque([])

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7️⃣ Set

s = set()
s.add(1)
1 in s
s.remove(1)

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8️⃣ Dictionary (HashMap)

mp = {}
mp["a"] = 1

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📌 Access

mp["a"]
"a" in mp

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📌 Loop

for key in mp:
for val in mp.values():
for k, v in mp.items():

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📌 Dict Comprehension

{i: i*2 for i in range(3)}

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9️⃣ Tuple

t = (1, 2)

• Immutable
• Can be used as hashmap key

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🔟 Heap (Priority Queue)

import heapq
 
minHeap = []
heapq.heappush(minHeap, 3)
heapq.heappop(minHeap)

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📌 Max Heap Trick

heapq.heappush(maxHeap, -val)

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📌 Heapify

heapq.heapify(arr)

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1️⃣1️⃣ Functions

def add(a, b):
    return a + b

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📌 Nested Functions

def outer():
    x = 10
    def inner():
        return x

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⚠️ Nonlocal

def outer():
    x = 1
    def inner():
        nonlocal x
        x += 1

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1️⃣2️⃣ Classes

class MyClass:
    def __init__(self, nums):
        self.nums = nums
 
    def size(self):
        return len(self.nums)

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🔥 Final Interview Tips

• Use:
  • list → stack
  • deque → queue
  • set → fast lookup
  • dict → hashmap
  • heapq → priority queue

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