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List of Useful Python Tricks for Solving Coding Questions

Saad Bash

Python is great language choice when solving DSA questions because of its english-like syntax, rich set of built-in libraries.

Here is list of some helpful methods and tricks when solving DSA questions.

collections.Counter()

  • Elements as keys, and their counts are stored as values.
nums = [1,1,2,3,4,5,3,2,3,4,2,1,2,3]

counter = collections.Counter(nums)
print(counter)
# Output: Counter({2: 4, 3: 4, 1: 3, 4: 2, 5: 1})

reversed(range)

for i in reversed(range(5)):
    print(i)
# Output
# 4
# 3
# 2
# 1
# 0

# Iterate list in reversed order
my_list = [1, 2, 3]
for i in reversed(range(len(my_list))):
    print(my_list[i])
# Output
# 3
# 2
# 1

defaultdict(set) and defaultdict(list)

defaultdict is a subclass of the built-in dict class in Python. The functionality of both dictionaries and defaultdict are almost same except for the fact that defaultdict never raises a KeyError.

Examples:

from collections import defaultdict

# Create a defaultdict with a set as the default factory function
d = defaultdict(set)

# Add some values to the defaultdict
d['key1'].add('value1')
d['key1'].add('value2')
d['key2'].add('value3')

# Access a key that doesn't exist yet => No KeyError
print(d['key3'])  # Output: set()

# Access an existing key
print(d['key1'])  # Output: {'value1', 'value2'}
from collections import defaultdict

# Create a defaultdict with a list as the default factory function
d = defaultdict(list)

# Add some values to the defaultdict
d['key1'].append('value1')
d['key1'].append('value2')
d['key2'].append('value3')

# Access a key that doesn't exist yet
print(d['key3'])  # Output: []

# Access an existing key
print(d['key1'])  # Output: ['value1', 'value2']

Create empty list of certain size

  1. Creating an empty list

    l = [None] * 10

  2. With zeros

    x = [0] * 5

collections.deque([iterable, [maxlen]]) - Doubly Ended Queue

  • For faster O(1) append and pop operations on both ends
queue = collections.deque([1, 2, 3])

## Operations
queue.append(4)
queue.appendleft(0)

queue.pop() # Returns 4
queue.popleft() # Returns 0

enumerate with optional start argument

Specify the starting index for the keys (indexes) in the enumerate object

name="saad"
for i, c in enumerate(name,start=1):
    print(f'{i}->{c}')

# Output
# 1->s
# 2->a
# 3->a
# 4->d

String strip()

strip() method removes any leading and trailing whitespaces from a given string.

text = "  Hello, World     "
stripped = text.strip()

print(stripped) # "Hello, World"

String isalnum()

isalnum() method returns True if all characters in the string are alphanumeric (either alphabets or numbers). If not, it returns False.

print(" ".isalnum()) # False

x = "123"
print(x.isalnum()) # True

y = "hello1"
print(y.isalnum()) # True

Create Positive & Negative Infinity

# Positive
x = float('inf')

# Negative
y = -float('inf')

# OR using math module
import math
x = math.inf
y = -math.inf