Python

Question 1

Strings mutable?

Answer 1

No, immutable. If you want to concat a bunch of strings without creating a new string for each intermediate operation, use ““.join(list)

Question 2

Returns true if all char are alphanumeric (and at least one char)

Answer 2

isalnum()

Question 3

Returns true if all char are letters (and at least one char)

Answer 3

isalpha()

Question 4

Returns true if all char are digits (and at least one digit)

Answer 4

isdigit()

Question 5

Returns a string which is a concatenation of the strings in a list (or other iterable)

Answer 5

join
““.join

Question 6

Split a string on whitespace, ignore empty strings

Answer 6

split no param
That means splitting an empty string will return an empty list

Question 7

Split a string on whitespace, include empty string

Answer 7

split(‘ ‘)

Question 8

Remove leading and trailing whitespace

Answer 8

strip

Question 9

Add an element to list

Answer 9

append
If you’re trying to create a list of lists, use append rather than +=

Question 10

Combine two lists

Answer 10

+ or += joins two lists into a single list
append adds a single element to a list. that’s why you want to use append to compose a list of lists.
extend is an in-place function call so you should return the original list

Question 11

set operations

Answer 11

set()
add, remove

Question 12

list pop no argument

Answer 12

removes the last item in the list

Question 13

get k,v from map

Answer 13

items()
Returns a dict_items object which is not accessible via index.
Need to use operator.getitem()

Question 14

sort

Answer 14

list.sort(). returns None. operates on a list, in place, stable, does not return new list b/c it’s an in place sort, key arg takes a fn that extracts a comparison key

Question 15

convert comparator to key

Answer 15

func_tools.cmp_to_key

Question 16

sorted

Answer 16

sorted(list)
built in method.
returns new sorted list.
stable.
key arg takes a function that extracts a comparison key from a single element.
sorted(list, key=len)
sorted(list, key=abs)

Question 17

class defn

Answer 17

The purpose of the init method is to initialize the object’s attributes. Used in classes only.

class Person:
def __init__(self, name, age):
self.name = name
self.age = age

Question 18

return the key corresponding to the max val in a freq map

Answer 18

max(freq_map.items(), key=operator.itemgetter(1))[0]

Question 19

f = operator.itemgetter(2)
what does f(‘ABC’) return

Answer 19

Returns C, the item at index 2

Question 20

g = operator.itemgetter(2, 5, 3)
what does g(‘ABCDEF’) return?

Answer 20

Returns a list with the items at indicies 2, 5, and 3
(‘C’, ‘F’, ‘D’)

Question 21

initialize dict

Answer 21

defaultdict(int)

Question 22

remove first element of list

Answer 22

list.pop(0)
O(n)!
Use deque instead

Question 23

reverse a list

Answer 23

list[::-1]
list.reverse() does it in place. can’t assign a return value.
list.reversed() returns a new reversed list.

Question 24

pop vs get for map

Answer 24

pop will throw a KeyError is no default is provided and the key is not found

Question 25

max int

Answer 25

MAX SIZE

from sys import maxsize

sys.maxsize or sys.maxsize - 1

Question 26

sort ascending

Answer 26

list.sort()
In place

Question 27

l1.extend(l2)

Answer 27

Adds the second list to the first list in place. Returns None. Need to return l1 for the combined list.

Question 28

python deque

Answer 28

from collections import deque
deque()
deque.append()
deque.appendleft()
can’t pop from empty deque! use len to check
deque.pop()
deque.popleft()

Question 29

Initialize array fixed value

Answer 29

dp = [None] * n
dp = [float(‘inf’)] * n
dp = [float(‘-inf’)] * n

Question 30

How do you solve dynamic programming problems?

Answer 30

• Use an array to capture state. So dp[i] represents the state of the system at i. The number of steps require to get to the ith step. The minimum number of coins needed to make the amount i. The length of the longest subsequence ending with the ith element.
• Determine a function to transition between elements. If know dp[i], how do I calculate dp[i+i]?
• Base case. How do we get started? Usually one or zero but can be tricky.

Question 31

How do you identify dynamic programming problems?

Answer 31

• Asking for max or min of something, particularly the max or min number of combinations (but not all max/min problems require dynamic programming e.g. stock trade)
• Having to calculate state based on a previous state (longest subsequence, for example)

Question 32

import types such as List

Answer 32

from typing import List

Question 33

are collections hashable? can they be used as a key in hashmaps?

Answer 33

No. Use tuple instead.

Question 34

create list from dict_values

Answer 34

list(dict_values)

Question 35

create defaultdict

Answer 35

from collections import defaultdict

defaultdict(int)
defaultdict(float)
defaultdict(str)
defaultdict(list)

can do other fancier things like a lambda function

Question 36

call class

Answer 36

s = Solution()
s.method()

Question 37

import Counter

Answer 37

from collections import Counter

Question 38

pass func to sort

Answer 38

list.sort(lambda x: x[0])

Question 39

Combine a list of lists

Answer 39

Use + instead of append!

Question 40

Sort descending

Answer 40

list.sort(reverse=True)

Question 41

How many unicode characters are there?

Answer 41

More than a million

Question 42

how do you convert the right-most bit to zero?

Answer 42

AND n & n-1

Question 43

Binary Search Time Complexity

Answer 43

O(log n)
Collection must be ordered

Question 44

divmod

Answer 44

divmod(n, 3) returns (result of division by 3, remainder)

Question 45

heap operations

Answer 45

Import heapq

heap = []
Heapq.heapify(heap)
Heapq.heappush(h, value)
Heapq.heappop(h)
while heap
nlargest
nsmallest

Question 46

heap[0]

Answer 46

smallest value
python heap implementation is min heap

Question 47

how to think about dict.items

Answer 47

iterable (k,v) tuple
basically only good for for (k,v) in dict
can also use with max/min/sort(key=operator.itemgetter(0 or 1))[0 or 1]
not itemgetter has no _
no itemgetter is a fn that uses ( not [

Question 48

how do you delete a key in a dict

Answer 48

del dict[key] if you know the key exists
dict.pop(key) throws error is key does not exist
dict.pop(key, None) return key if it exists. Doesn’t throw error if the key doesn’t exist.

Question 49

dict.setdefault(key, value)

Answer 49

• returns the value of the item with the specified key
• if the key does not exist, insert the key, with the specified value