Python Data Structures + Algorithms

Question 1

What is an array?

Answer 1

an ordered collection of elements

Question 2

What is the time complexity of insertion/deletion of an array?

Answer 2

O(1)

Question 3

What is the time complexity of searching an array?

Answer 3

O(n)

Question 4

How do I create a list?

Answer 4

list() or my_list = [1, 2, 3]

Question 5

How do i add elements to a list?

Answer 5

append, insert(num, idx)

Question 6

How do I remove the last element of a list?

Answer 6

my_list.pop()

Question 7

How do I deleted an element from a list?

Answer 7

del my_list[2]

Question 8

What is unique about arrays in Python?

Answer 8

it is dynamic

Question 9

What is a string?

Answer 9

an immutable sequence of characters

Question 10

What is the time complexity of access on a string?

Answer 10

O(1)

Question 11

What is the time complexity of concatenation on a string?

Answer 11

O(n)

Question 12

How do I slice a string?

Answer 12

s[1:4]

Question 13

How do I concatenate a string?

Answer 13

“a” + “b”

Question 14

What is another name for a dictionary?

Answer 14

a hash map

Question 15

What is a dictionary?

Answer 15

a collection of key-value pairs

Question 16

Why do we choose dictionaries?

Answer 16

they provide efficient lookups based on keys

Question 17

What is the access/insertion/deletion time complexity of a dictionary?

Answer 17

O(1)

Question 18

How do I create a dictionary?

Answer 18

{} or {“apple”: 1}

Question 19

How do I insert a new value to my dictionary?

Answer 19

dictionare[“banana”] = 2

Question 20

What are arrays used for in leetcode?

Answer 20

problems involving sequences, searching, sorting, and dynamic programming

Question 21

What are dictionaries used for in leetcode?

Answer 21

problems involving frequency, counting, lookings, and storing mapping between elements

Question 22

What is a set?

Answer 22

unordered collection of unique elements

Question 23

What is the time complexity for a set?

Answer 23

O(1)

Question 24

How do I create a set?

Answer 24

s = (1, 2, 3) or s = set([1, 2, 3])

Question 25

How do I add an element to a set?

Answer 25

s.add(4)

Question 26

How do I remove an element from a set?

Answer 26

s.remove(2)

Question 27

How do I check if something is in my set?

Answer 27

return 3 in s

Question 28

What are sets useful for?

Answer 28

problems that require checking for the existence of elements, finding unique elements and set operations

Question 29

What is a stack?

Answer 29

Last-In-First-Out data structure

Question 30

What is the time complexity of push/pop/peek of a stack?

Answer 30

O(1)

Question 31

How do I create a stack?

Answer 31

my_stack = []

Question 32

How do I add to my stack?

Answer 32

my_stack.append(1)

Question 33

How do I remove the top element of my stack?

Answer 33

my_stack.pop()

Question 34

What leetcode problems are stacks used for?

Answer 34

problems that involve backtracking, parsing expressions, and DFS

Question 35

What is a queue?

Answer 35

First-In-First-Out data structure

Question 36

What is the time complexity of enqueue/dequeue?

Answer 36

O(1)

Question 37

How do we use a queue in python?

Answer 37

by using from collections import deque

Question 38

How do I instantiate a deque?

Answer 38

my_queue = deque()

Question 39

How do I append to a queue?

Answer 39

my_q.append(1)

Question 40

How do I pop from my queue?

Answer 40

my_q.popleft()

Question 41

What leetcode problems are queues used for?

Answer 41

BFS and level-order traversal

Question 42

What are linked lists?

Answer 42

sequence of nodes where each node contains data and a pointer to the next node

Question 43

What is a heap?

Answer 43

a binary heap for min/max priority queues

Question 44

What is the time complexity of a heap insert and pop?

Answer 44

O(log n)

Question 45

How do we begin to use a heap?

Answer 45

import heapq

Question 46

How do we instantiate a heap?

Answer 46

heap = []

Question 47

How do we add to a heap?

Answer 47

heapq.heappush(heap, 3)

Question 48

How do we pop from a heap?

Answer 48

heapq.heappop(heap)

Question 49

What is the time complexity of insertion/deletion of a linked list?

Answer 49

O(1)

Question 50

What is the time complexity of linked list insertion and deletion in middle or end?

Answer 50

O(n)

Question 51

How do I create the linked list class?

Answer 51

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

Question 52

How do I create a head for my linked list?

Answer 52

head = ListNode(1)

Question 53

How do I add a new node to my existing linked list?

Answer 53

head.next = ListNode(2)

Question 54

What are linked lists useful for in leetcode problems?

Answer 54

dynamic memory allocation, insertion/deletion at arbitrary positions, and representing sequences where size is not known beforehand

Question 55

What is a graph?

Answer 55

collection of nodes connected by edges. can be directed, undirected, weighted or unweighted

Question 56

What is an example of a graph in python?

Answer 56

graph = {0: [1, 2], 1: [0], 2: [0]}

Question 57

What is a Binary Tree?

Answer 57

a hierarchical data structure consisting of nodes connected by edges

Question 58

What is different about a Binary Search Tree?

Answer 58

it maintains a sorted order

Question 59

What is the time complexity of access/search/insert/delete of a balanced BST?

Answer 59

O(log n)

Question 60

What is the worst case time complexity of a BST

Answer 60

when it is unbalances; O(n)

Question 61

How do we create a tree class?

Answer 61

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

Question 62

How do we instantiate a tree?

Answer 62

root = TreeNode(5)

Question 63

How do we add a new node to a tree?

Answer 63

root.left = TreeNode(3)

Question 64

What are trees used for in leetcode problems?

Answer 64

problems involving searching, sorting, hierarchical relationships, lookups

Question 65

What does heap allow efficient retrieval of?

Answer 65

the smallest or largest element

Question 66

What are heaps useful for in leetcode problems?

Answer 66

finding the k-th smallest/largest element, scheduling, Dijkstra's

Question 67

What is the space complexity of graphs?

Answer 67

O(V + E)

Question 68

What are leetcode problems are graphs useful for?

Answer 68

relationships between entities, pathfinding, network analysis