Leetcode Tips n Tricks

Question 1

What’s a cleaner way of writing the code below?

my_dict = {}
if value in my_dict:
    my_dict[value].append('hi')
else:
    my_dict[value] = ['hi']

Answer 1

from collections import defaultdict
my_dict = defaultdict(list)
my_dict[value].append('hi')

defaultdict will return the default value instead of returning a KeyError

Question 2

How could you represent a letter as an integer?

Answer 2

Use ord(letter), which will return the unicode integer for that letter

Question 3

Create a list of 3 zeroes

Answer 3

zeroes = [0] * 3

Question 4

How do you iterate backwards on a list?

Answer 4

for i in reversed(a):

Question 5

Explain why my_l_of_l = [[]] * 10 will NOT work in making a list of empty lists?

Answer 5

Because this will only make a list of the same list object!

Question 6

How do you make a list of empty lists?

Answer 6

my_l_of_l = [[] for _ in range(n)]

Question 7

If you weren’t allowed to use collections.Counter to create a counting dictionary, how would you do it?

Answer 7

my_list = [1, 2, 2, 2, 3, 3]
count_dict = {}
for val in my_list:
    count_dict[val] = 1 + count_dict.get(val, 0)