Python Reference

# Lists
nums = [1, 2, 3]  # [1, 2, 3]
nums[0]  # 1
nums[-1]  # 3
nums[1:3]  # [2, 3]
nums[::-1]  # [3, 2, 1]

# Strings
s = "abc"
s[0]  # 'a'
s[1:]  # 'bc'
s[::-1]  # 'cba'

# Dict
d = {"a": 1}
d["a"]  # 1
d.get("b", 0)  # 0

# Set
seen = set()
seen.add(3)
3 in seen  # True

# Looping
for i in range(3):
    pass
# i goes 0, 1, 2

for i in range(1, 4):
    pass
# i goes 1, 2, 3

for i in range(5, 0, -1):
    pass
# i goes 5, 4, 3, 2, 1

# enumerate
for i, x in enumerate(["a", "b"]):
    pass
# (i, x) = (0, 'a'), (1, 'b')

# zip
list(zip([1, 2], [3, 4]))  # [(1, 3), (2, 4)]

# Conditionals
x = 5
if x > 0:
    y = "pos"
else:
    y = "non-pos"
y  # 'pos'

# Tuple swap
a, b = 1, 2
a, b = b, a
(a, b)  # (2, 1)

# Integer division and modulo
7 // 3  # 2
7 % 3  # 1

# Reverse loop often used for carries / reverse scanning
list(range(3, -1, -1))  # [3, 2, 1, 0]

# len / sum
len([1, 2, 3])  # 3
sum([1, 2, 3])  # 6
min(4, 2)  # 2
max(4, 2)  # 4
sorted([3, 1, 2])  # [1, 2, 3]

# any / all
any([False, True, False])  # True
all([True, True, False])  # False

# list methods
arr = [3, 1, 2]
arr.append(4)
arr  # [3, 1, 2, 4]
arr.pop()
# 4
arr  # [3, 1, 2]
arr.reverse()
arr  # [2, 1, 3]

# string methods
"a".isalnum()  # True
"!".isalnum()  # False
"A".lower()  # 'a'
"7".isdigit()  # True
"abc".find("bc")  # 1

# ord
ord("0")  # 48
ord("7") - ord("0")  # 7

# bit operations
0b101 # 5
bin(5) # "0b101"
format(5, "b") # "101"
format(5, "08b") # "00000101"
5 & 1  # 1
4 & 1  # 0
bin(0b101 | 0b10)  # 0b111
bin(0b101 ^ 0b111)  # 0b10
bin(0b101 << 1)  # 0b1010
bin(0b100 << 1)  # 0b1000
bin(0b101 >> 1)  # 0b10
bin(0b100 >> 1)  # 0b10

# Python 3.8+ int bit count
bin(11) # "0b1011"
(11).bit_count()  # 3

# randrange
from random import randrange

randrange(10)      # 0 to 9
randrange(1, 10)   # 1 to 9
randrange(0, 20, 2)  # even number from 0 to 18

# shuffle
from random import shuffle

items = [1, 2, 3, 4, 5]
shuffle(items)
items # list in random order

# deque
from collections import deque

q = deque([1, 2, 3])
q.appendleft(0)
q # deque([0, 1, 2, 3])
q.popleft()  # 0
list(q) # [1, 2, 3]

# Slice assignment mutates same list object
a = [1, 2, 3, 4]
a[:] = [5, 6]
a  # [5, 6]
a = [1, 2, 3, 4]
a[1:3] = [5, 6]
a  # [1, 5, 6, 4]

# Useful for LeetCode in-place APIs
nums = [1, 2, 3, 4]
nums[:] = nums[::-1]
nums  # [4, 3, 2, 1]

# Counter intersection
from collections import Counter

Counter([1,2,2,2,3,4]) # Counter({2: 3, 1: 1, 3: 1, 4: 1})
Counter([2,2]) # Counter({2: 2})
Counter([1,2,2,2,3,4]) & Counter([2,2]) # Counter({2: 2})
list((Counter([1,2,2,2,3,4]) & Counter([2,2])).elements()) # [2, 2]

# Reverse loop for carry / right-to-left scans
list(range(2, -1, -1))  # [2, 1, 0]

# Common leetcode helpers

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right