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Array, Hash

These are "design a class" problems where the right internal state is a plain list or an index trick rather than anything fancy. Mapping account numbers, car types, or stream keys onto array indices keeps every operation simple and fast.

General​

2043. Simple Bank System​

Medium·
FIG. SIMPLE BANK SYSTEM● INTERACTIVE
visualization loads as you reach it
class Bank:
def __init__(self, balance: List[int]):
self.balance = balance
 
def transfer(self, account1: int, account2: int, money: int) -> bool:
if self.withdraw(account1, money):
if self.deposit(account2, money):
return True
self.deposit(account1, money)
return False
 
def deposit(self, account: int, money: int) -> bool:
if self.is_valid_account(account):
self.balance[account - 1] += money
return True
return False
 
def withdraw(self, account: int, money: int) -> bool:
if self.is_valid_account(account) and self.balance[account - 1] >= money:
self.balance[account - 1] -= money
return True
return False
 
def is_valid_account(self, account: int) -> bool:
return 1 <= account <= len(self.balance)

1603. Design Parking System​

Easy·
FIG. DESIGN PARKING SYSTEM● INTERACTIVE
visualization loads as you reach it
class ParkingSystem:
def __init__(self, big: int, medium: int, small: int):
self.availability = [None, big, medium, small]
 
def addCar(self, carType: int) -> bool:
if self.availability[carType] > 0:
self.availability[carType] -= 1
return True
return False

1476. Subrectangle Queries​

Medium·
3 Approaches
FIG. SUBRECTANGLE QUERIES AS IS● INTERACTIVE
visualization loads as you reach it
class SubrectangleQueries:
def __init__(self, rectangle: List[List[int]]):
self.rectangle = rectangle
 
def iterate_over(self, row1: int, col1: int, row2: int, col2: int) -> Tuple[int, int]:
for r in range(row1, row2 + 1):
for c in range(col1, col2 + 1):
yield r, c
 
def updateSubrectangle(self, row1: int, col1: int, row2: int, col2: int, newValue: int) -> None:
for row, col in self.iterate_over(row1, col1, row2, col2):
self.rectangle[row][col] = newValue
 
def getValue(self, row: int, col: int) -> int:
if 0 <= row < len(self.rectangle) and 0 <= col < len(self.rectangle[0]):
return self.rectangle[row][col]

1656. Design an Ordered Stream​

Easy·
FIG. DESIGN AN ORDERED STREAM● INTERACTIVE
visualization loads as you reach it
class OrderedStream:
def __init__(self, n: int):
self.pointer = 0
self.data = [None for i in range(n)]
self.n = n
 
def insert(self, idKey: int, value: str) -> List[str]:
idKey = idKey - 1 # 0-indexing
self.data[idKey] = value
while self.pointer < self.n and self.data[self.pointer]:
self.pointer += 1
return self.data[idKey : self.pointer]

271. Encode and Decode Strings​

Medium·
3 Approaches
class Codec:
def encode(self, strs):
"""Encodes a list of strings to a single string."""
return "π".join(strs)
 
def decode(self, s):
"""Decodes a single string to a list of strings."""
return s.split("π")