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Python Fundamentals for LeetCode & DSA Interviews

Python’s expressive syntax and rich standard library make it ideal for coding interviews.

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1. Comprehensions & Unpacking

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List Comprehensions

arr = [1, 2, 3, 4]
evens = [x for x in arr if x % 2 == 0]    # [2, 4]
squares = [x*x for x in arr]             # [1, 4, 9, 16]

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Dict & Set Comprehensions

pairs = [('a',1), ('b',2)]
d = {k: v for k,v in pairs}               # {'a':1, 'b':2}
unique = {x for x in arr if x%2==0}       # {2, 4}

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Tuple Unpacking

x, y = 5, 10
x, y = y, x       # swap: x=10, y=5
a, b, *rest = [0, 1, 2, 3]  # a=0, b=1, rest=[2, 3]

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Generators

gen = (x*x for x in range(100))  # generator expression

def gen_range(n):
    i = 0
    while i < n:
        yield i
        i += 1

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2. Iteration & Functional Tools

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enumerate()

Definition: enumerate() returns pairs of (index, value) from an iterable, allowing you to loop with both the index and the value.

for i, val in enumerate(['a','b','c'], start=1):
    print(i, val)

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zip()

Definition: zip() aggregates elements from two or more iterables (like lists or tuples), returning tuples with one element from each iterable.

a = [1,2,3]; b = ['x','y','z']
for num, ch in zip(a, b):
    print(num, ch)

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map() and filter()

Definition: map() applies a function to every item in an iterable and returns a map object (which can be converted to a list). filter() filters items in an iterable based on a function that returns True or False.

doubles = list(map(lambda x: x*2, arr))
odds = list(filter(lambda x: x%2, arr))

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any() and all()

Definition: any() returns True if at least one element in the iterable is true. all() returns True if all elements in the iterable are true.

any(x < 0 for x in arr)
all(x is not None for x in arr)

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sorted(), reversed(), and sum()/min()/max()

Definition: sorted() returns a new sorted list from the items in an iterable. reversed() returns an iterator that accesses the given sequence in the reverse order. sum(), min(), and max() return the sum, minimum, and maximum of an iterable, respectively.

sorted(arr)
sorted(arr, reverse=True)
sum(arr); min(arr); max(arr)
list(reversed(arr))

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range()

Definition: range() generates a sequence of numbers, commonly used for looping a specific number of times in for-loops.

  • Remember: range(n) → 0..n-1

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itertools.chain()

Definition: itertools.chain() takes multiple iterables and returns a single iterator that produces items from the first iterable until it is exhausted, then continues to the next iterable, and so on.

from itertools import chain
nested = [[1, 2], [3, 4]]
flat = list(chain.from_iterable(nested))  # [1,2,3,4]

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3. Data Structures & Libraries

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collections.Counter

Definition: collections.Counter is a dict subclass for counting hashable objects. It returns a dictionary where elements are stored as keys and their counts as values.

from collections import Counter
cnt = Counter([1,2,2,3,1,1])
cnt.most_common(2)  # [(1, 3), (2, 2)]

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collections.defaultdict

Definition: collections.defaultdict is a dict subclass that provides a default value for nonexistent keys, avoiding KeyError.

from collections import defaultdict
group = defaultdict(list)
for k,v in pairs:
    group[k].append(v)

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collections.deque

Definition: collections.deque is a double-ended queue that supports fast appends and pops from both ends.

from collections import deque
q = deque([start])
while q:
    node = q.popleft()
    q.append(next_node)

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heapq (min-heap)

Definition: The heapq module provides an implementation of the min-heap queue algorithm, also known as the priority queue algorithm.

import heapq
heap = []
heapq.heappush(heap, 5)
heapq.heappush(heap, 1)
heapq.heappop(heap)  # 1

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bisect

Definition: The bisect module provides support for maintaining a list in sorted order without having to sort the list after each insertion.

import bisect
a = [1,3,4,7]
i = bisect.bisect_left(a, 5)
a.insert(i, 5)  # [1,3,4,5,7]

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4. Pythonic Idioms & Techniques

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Sorting with key=

people = [('alice',25), ('bob',20)]
people.sort(key=lambda x: x[1])  # Sort by age
min(people, key=lambda x: x[1])

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Lambda expressions

lambda x: x[1]

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enumerate() + zip() unpacking

for idx, (x, y) in enumerate(pairs):
    ...

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functools.lru_cache for memoization

Definition: functools.lru_cache is a decorator that caches the results of function calls, so that when the same inputs occur again, the cached result is returned instead of recomputing.

from functools import lru_cache

@lru_cache(maxsize=None)
def fib(n):
    ...

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5. Common Pitfalls

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Mutable Default Arguments

# Bad
def f(a=[]): a.append(1); return a

# Good
def f(a=None):
    if a is None:
        a = []
    a.append(1)
    return a

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Shallow vs Deep Copy

a = [[1,2],[3,4]]
b = list(a)          # Shallow copy
b[0][0] = 9
# a becomes [[9,2],[3,4]] — wrong!

import copy
b = copy.deepcopy(a)  # Deep copy

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Off-by-One Errors

  • range(n) is 0..n-1
  • range(len(arr)) is correct
  • Be careful with <= vs <

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Integer Division

5 / 2  # 2.5
5 // 2  # 2 (correct for integer division)

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List Multiplication Pitfall

# Wrong
matrix = [[0]*m]*n

# Correct
matrix = [[0]*m for _ in range(n)]

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is vs ==

a == b  # value equality
a is b  # identity (same object)

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6. Best Practices

  • Use comprehensions ([x for x in ...])
  • Prefer built-ins: Counter, min, max, sum, sorted
  • Use defaultdict for grouping
  • Use heapq.nlargest / nsmallest for “top K” problems
  • Sort with key= function
  • Cache expensive calls with @lru_cache
  • Avoid unnecessary loops with map, filter, zip, enumerate
  • Keep code readable and Pythonic