Python Programming Practice: LeetCode #1 -- Two Sum

In this episode of Python Programming Practice, we tackle LeetCode #1 -- Two Sum.

Link to the problem here:

Python Programming Practice is a series focused on teaching practical coding skills by solving exercises on popular coding websites. Note that the solutions seen here may not be the most efficient possible.

I am not going to provide the full code in the video description for this series, since copy and pasting solutions is not in the spirit of doing coding exercises. It is intended that the video will help you think about problems, approaches and how to structure solutions so that you are able to code up a working solution yourself.

[GamingNoodle]

I've seen 100 vids on this two-sum problem and it just never clicked with me. I just dont think people could teach it in a way that I would understand. This short video of yours very clearly explained it and I finally understand now. This was a big brick wall for me so it feels amazing to finally get through it.

[maxgriffiths]

This is great content. Your explanations are clear and logical. Thanks

[NOBOBI]

def twoSum(self, nums: List[int], target: int) -> List[int]:
numToIndex = {}
for i in range(len(nums)):
if target - nums[i] in numToIndex:
return [numToIndex[target - nums[i]], i]
numToIndex[nums[i]] = i
return []

[VihaanKembhavi-uy]

def twoSum(array, target):
for x in array:
if target - x in array and target - x != x: return [array.index(x), array.index(target - x)]

easy 3 liner

[kvelez]

def sum_index(arr, i):
if i >= len(arr) - 1:
return 0
now = arr[i]
next = arr[i+1]
sum = now+next
print(f"Sum {i+1}: {sum}")
return sum + sum_index(arr, i+1)
print(sum_index([2, 7, 11, 18], 0))

[Draco-puro]

Your explanations are so precise and easy to understand. Is it possible to make three sum explanation video?

[jimmymesa]

In the 2nd solution, how does Python find out whether the value is or is not in the dictionary without looping through the dictionary somehow?

[joshikaran-zmju]

other videos in yt looks like they memorizing the code and faking like the they thinking and coding, but this one looks legit

[marcofer]

Great video with clear explanation on every concept. Amazing!

[jaishreeram]

Using recursion:
def twoSum(j=0, i=1):
if (nums[j]+nums[i])==target:
return [j, i]
else:
return twoSum(j+1, i+1)

print(twoSum())

[liatris]

What about traversing the list, subtracting the number at the current index from 9 and then searching the list for the complement?

[boy]

This is an awesome walkthrough of coding !! Just a quick question.. Do we need the elif in 2nd solution ? Can't it be within for .

[yowanehakuuu]

Amazing explanation, I understood for the first time after watching many videos

[Kerogas_]

I don't understand the 2nd solution.
At first we have an empty dict.
On a first iteration we added "2: 0" to it.
On a second iteration we subtracted 7 from 9 (target - num) and got 2, that we already have in "seen" as key.
But what is going on in the line 8???
As I can see it is [2, 1] because 9-7 = 2 and i on 2nd iteration is 1.
But how are we getting [0, 1]?

[Elunicozorro]

Should we just assume that for the first solution that the two values that sum to target will be consecutive?

[ajmo]

def check_result(myList, target):

if len(myList) <= 1 :
return myList
else:

list_indx = []

for i in range(len(myList)-1):
if myList[i] + myList[i+1] > target \
or target != myList[i] + myList[i+1] :

continue
list_indx.extend([i, i+1])

return list_indx


myList = [1, 2, 3, 4, 5, 6]

target = 7
print(check_result(myList, target ))

[michasekua]

I added this code into my Pycharm.

nums = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 100]
target = 18

class Solution:
def twoSum(self, nums: List[int], target: int) -> List[int]:
seen = {}
for i, num in enumerate(nums):
if target - num in seen:
return([seen[target - num], i])
elif num not in seen:
seen[num] = i

print(twoSum())

and I got error:
NameError: name 'List' is not defined

[bvlmari]

Great explanation, i needed it thank you!

[TheMouseJerry-dumd]

Thank you for sharing the videos. keep going and I'm purely subscribing to your channel just for leet code problems in python.

[ThatWomanIsNotMeAndImNotWhite]

The second solution was highly efficient