I will be documenting my learning journey in Data Structures and Algorithms here. I will be using C++ / Python / Java for this purpose.

I am trying to follow Striver's A2Z DSA Course/Sheet. It's extremely organised, starts from the fundamentals and goes on to advanced topics. I will be following this course and also supplementing it with other resources like GeeksforGeeks and LeetCode.

I hope to make a commit everyday as I learn one new thing everyday. I will be taking notes in Markdown and also include C++ files where necessary.

I hope that this public contract will keep me accountable and help me stay consistent in my learning.

1. https://www.hackerrank.com/challenges/solve-me-first/problem (5 Feb 2025)
2. https://www.hackerrank.com/challenges/cpp-hello-world/problem?isFullScreen=true (5 Feb 2025)
3. https://www.hackerrank.com/challenges/c-tutorial-basic-data-types/problem?isFullScreen=true (5 Feb 2025)
4. https://www.hackerrank.com/challenges/cpp-input-and-output/problem?isFullScreen=true (9 Feb 2025)
5. https://www.hackerrank.com/challenges/c-tutorial-for-loop/problem?isFullScreen=true (12 Feb 2025)
6. https://www.hackerrank.com/challenges/c-tutorial-conditional-if-else/problem?isFullScreen=true (12 Feb 2025)
7. https://www.hackerrank.com/challenges/c-tutorial-functions/problem?isFullScreen=true (12 Feb 2025)
8. https://www.hackerrank.com/challenges/c-tutorial-pointer/problem?isFullScreen=true
9. https://www.hackerrank.com/challenges/arrays-introduction/problem?isFullScreen=true
10. https://www.hackerrank.com/challenges/python-division/problem?isFullScreen=true [Python] (23 Mar 2025)
11. https://www.hackerrank.com/challenges/python-loops/problem?isFullScreen=true [Python] (23 Mar 2025)
12. https://www.hackerrank.com/challenges/python-arithmetic-operators/problem?isFullScreen=true [Python] (24 Mar 2025)
13. https://www.hackerrank.com/challenges/python-print/problem?isFullScreen=true [Python] (24 Mar 2025)
14. https://www.hackerrank.com/challenges/python-integers-come-in-all-sizes/problem?isFullScreen=true [Python] (24 Mar 2025)
15. https://www.hackerrank.com/challenges/python-power-mod-power/problem?isFullScreen=true [Python] (24 March 2025)
16. https://www.hackerrank.com/challenges/python-power-mod-power/problem?isFullScreen=true [Python] (24 March 2025)
17. https://www.hackerrank.com/challenges/np-sum-and-prod/problem?isFullScreen=true [Python, Numpy] (25 March 2025)
18. https://www.hackerrank.com/challenges/bash-tutorials---the-world-of-numbers/problem?isFullScreen=true [Bash] (28th March 2025)
19. https://www.hackerrank.com/challenges/swap-case/problem?isFullScreen=true [Python, sWAP cASE] (2nd April 2025)
20. https://www.hackerrank.com/challenges/whats-your-name/problem?isFullScreen=true [Python, Basic Printing] (4th April 2025)
21. https://www.hackerrank.com/challenges/simple-array-sum/problem?isFullScreen=true [Array Sum] (9th Apr 2025)
22. https://www.hackerrank.com/challenges/py-introduction-to-sets/problem?isFullScreen=true [Python, Sets] (10th Apr 2025)
23. https://www.hackerrank.com/challenges/conditional-statements-in-c/problem?isFullScreen=true [If-Else in C] (12th Apr 2025)
24. https://www.hackerrank.com/challenges/find-a-string/problem?isFullScreen=true [Sliding Window Substring Matching] (13th Apr 2025)
    1. Learnt how to slide a window over a string to find a substring. Created https://claude.site/artifacts/d26622f9-69ef-4dfa-90da-303be1cea047 to visualize it.
    2. Trying to setup commitizen
25. https://www.hackerrank.com/challenges/py-set-add/problem?isFullScreen=true (15 Apr 2025)
    1. Learnt s.add() method in set for Python.
26. https://www.hackerrank.com/challenges/python-string-split-and-join/problem?isFullScreen=true
    1. Learnt about split() and join(). I knew split which splits a string into a list (" " is the default delimiter). However join() I didn't know too well. "-".join(a) where a is a list returns "a[0]-a[1]-a[2]...a[len(a)-1]"
27. https://www.hackerrank.com/challenges/python-lists/problem?isFullScreen=true [Python Lists]
    1. Slew of List functions like pop, append, l[::-1], .remove, .sort.
    2. sort sorts the list in-place.
28. https://www.hackerrank.com/challenges/compare-the-triplets/problem?isFullScreen=true
    1. Use zip which iterates over tuples.
    2. Basic if, elif and else logic.
29. https://www.hackerrank.com/challenges/bash-tutorials---a-personalized-echo/problem
    1. To get input in bash use the read command. Nice Bash Tutorial for beginners here.
30. https://www.hackerrank.com/challenges/text-processing-cut-1/problem
    1. cut -b <num> gets the <num> byte from a string.
31. https://www.hackerrank.com/challenges/find-second-maximum-number-in-a-list/problem?isFullScreen=true
    1. Sort the list, record the highest score. Keep on looping through the list from the end and check if the current value is equal to the highest score or not, if it is then that score is not the runner up. So continue. Else the current value is not equal to the highest score, then it must be the runner up as the list has already been sorted with the .sort() command which sorts a list in place in Python.

1. https://leetcode.com/problems/concatenation-of-array/description/ (18 Mar 2025)

2. https://leetcode.com/problems/return-length-of-arguments-passed/description/ (18 Mar 2025)

3. Height Checker: https://leetcode.com/problems/height-checker/description/?envType=problem-list-v2&envId=array (18 Mar 2025)

4. https://leetcode.com/problems/display-the-first-three-rows/description/ [Pandas] (18 Mar 2025)

5. https://leetcode.com/problems/modify-columns/description/ (20th Mar 2025)

6. https://leetcode.com/problems/rename-columns/description/ [Pandas] (20th Mar 2025)

7. https://leetcode.com/problems/change-data-type/description/ [Pandas] (20th Mar 2025)

8. https://leetcode.com/problems/select-data/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (22nd Mar 2025)

9. https://leetcode.com/problems/recyclable-and-low-fat-products/description/?envType=study-plan-v2&envId=top-sql-50 [SQL] (27th Mar 2025)

10. https://leetcode.com/problems/find-customer-referee/description/?envType=study-plan-v2&envId=top-sql-50 [SQL] (27th Mar 2025)

11. https://leetcode.com/problems/big-countries/?envType=study-plan-v2&envId=top-sql-50 [SQL] (27th Mar 2025)

12. https://leetcode.com/problems/article-views-i/description/?envType=study-plan-v2&envId=top-sql-50 [SQL] (27th Mar 2025)

13. https://leetcode.com/problems/create-a-dataframe-from-list/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (29th Mar 2025)

14. https://leetcode.com/problems/get-the-size-of-a-dataframe/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (29th Mar 2025)

15. https://leetcode.com/problems/create-a-new-column/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (29th March 2025)

16. https://leetcode.com/problems/drop-duplicate-rows/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (29th March 2025)

17. https://leetcode.com/problems/drop-missing-data/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (30th March) (nice editorial)

18. https://leetcode.com/problems/check-if-the-number-is-fascinating/description/ [Hash Table, Math] (30th March 2025)

19. https://leetcode.com/problems/number-of-good-pairs/ [DSA] (31st March 2025)

20. https://leetcode.com/problems/add-two-integers/description/ [Add 2 Numbers xD] (7 Apr 2025)

21. https://leetcode.com/problems/valid-palindrome/description/ [Palindrome] (7 Apr 2025)

22. https://leetcode.com/problems/intersection-of-two-arrays/description/ [Intersection of Two Arrays] (8 Apr 2025)

23. https://leetcode.com/problems/plus-one/ [Plus One] (9 Apr 2025)

24. https://leetcode.com/problems/shuffle-the-array/description/ [Shuffle an Array] (9 Apr 2025)

25. https://leetcode.com/problems/fill-missing-data/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata [Pandas] (11 Apr 2025)

26. https://leetcode.com/problems/running-sum-of-1d-array/description/ [Running Sum of 1D Array] (15 Apr 2025)

    1. Did a running sum of the array.

27. https://leetcode.com/problems/sqrtx/ (15 Apr 2025)

    1. Did this in O(n) time. Problem is to find the square root of a number without using built in functions. Can do this with binary search and thus reduce the problem into O(log(n))

28. https://leetcode.com/problems/reshape-data-concatenate/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata (16 April)

    1. Essentially teaches you how to use the pd.concat() function. concat() allows you to glue dataframes together. If axis=0 it does a row wise concatenation (increase the height of the dataframe) and if axis=1, it does a column wise concatenation (increase the width of the dataframe). Also supports join etc. i.e. inner and outer. Great short medium article here.

29. https://leetcode.com/problems/method-chaining/description/?envType=study-plan-v2&envId=introduction-to-pandas&lang=pythondata

    1. Method Chaining. Filter, sort the df by weight using sort_values(by='col_name') and then get the necessary column.

30. https://leetcode.com/problems/add-digits/description/?envType=problem-list-v2&envId=prshgx6i

    1. Take an integer, add all the digits of that integer to get a number. If that number has more than one digit, repeat.
    2. I created a modular solution whereby I created a function whose purpose was to input a number and output the sum of the digits of that number. Then I ran a while loop until the num//10 == 0 as that would mean that num is a number with one digit.

31. https://leetcode.com/problems/score-of-a-string/submissions/1643047539/

    1. Given a string, calculate the difference of the ASCII values of adjacent characters
    2. Use ord to get the ASCII value of a character in Python
    3. To get the absolute value of any number, use abs() instead of math.abs().

32. https://leetcode.com/problems/richest-customer-wealth/description/

    1. Return the max of the row wise sum of a mxn matrix

33. https://leetcode.com/problems/merge-strings-alternately/description/

    1. Given two strings s1 = "abc" and s2 = "pqr", merge them alternatively i.e. "apbqcr"
    • To solve:
      • Check which word has the least length
      • Run a for loop and add the characters till that least length i.e. do merged_str = merged_str + word1[i] + word2[i]
      • Then for the string that's longer (if that's even a case), add all the characters from len(shorter_str) to len(longer_word). Checkout my solution here for better context.

34. https://leetcode.com/problems/fizz-buzz/description/

    1. Classic FizzBuzz. Need I say more?

35. https://leetcode.com/problems/convert-the-temperature/description/

    1. Really basic temperature conversion problem. Kind of shocked how LeetCode has this up on their website.

36. https://leetcode.com/problems/first-unique-character-in-a-string/description/

    1. I iterate over the string. I push each character into a list. If a character is already in the list, I set it to False in a corresponding dictionary.
    2. Then I iterate over the string again and return the index of the first character that is True in the dictionary.
    3. It's a bit slow. I'm sure there's a better way to do this.

37. https://leetcode.com/problems/sum-multiples/description/

    1. Variant of FizzBuzz. First check if a number is divisible by 3, 5 or 7. If it is, add it to the sum. Then check if its divisible by both (3 and 5) or (3 and 7 [i didn't do this]) or (5 and 7). If it is, add it to the sum. Then check if its divisible each of the three. If it is, add it to the sum.
    2. Prevents overcounting i.e. Whereby you'd count a number twice if it's divisible by both 3 and 5.

38. https://leetcode.com/problems/find-closest-person/description/

    1. Really simple problem. 3 people. X, Y and Z. X and Y move towards Z at the same speed. It is our job to find out who will reach Z first. This can be simply solved by taking the difference.

39. https://leetcode.com/problems/maximum-number-of-words-found-in-sentences/description/

    1. Loop over each sentence in the list sentences. use .split() to get all the words and len() to get the number of words and then do a simple max_words if statement. Really easy problem.

40. [STRING] https://leetcode.com/problems/student-attendance-record-i/description/

    1. Loop over the string.
       1. First count the number of As using a dictionary. If this count is ever >=2 directly return False within the loop itself.
       2. Now we need to find a way to track if the string has 3 or more consecutive L. To do this, I use a variable called late_consec. Whenever the loop encounters a L this is incremented by 1. However, if the loop encounters a different letter i.e. a P or a A, then late_consec is reset to zero. This ensures that the only way late_consec can be more than 3 is if the loop encounters 3 or more Ls consecutively thereby making it impossible for the counter to be rest by any other the other if conditions.
       3. Within the if condition for the character L check if the counter is >=3 or not. If it is, then return False immediately, else keep on going.

I've solved 40 problems (atleast) that's how many I've recorded in this repository. I will now transition to doing more structured practice. So I asked o3 to give me a plan and it gave me this link: https://dev.to/dfs_with_memo/leetcode-warmup-problems-13o7. I will now use this list and try to solve problems using it. I will also try to tag my problems from now on.

41. [LINKED LIST] https://leetcode.com/problems/middle-of-the-linked-list/
    1. First get the length of the Linked List.
    2. Then calculate the middle index based on whether the length of the list is positive or negative.
    3. Then, traverse through the LL and keep a track of the number of the nodes you're visiting. The moment you hit the middle node (based on the index that was calculated earlier) save the reference of that node and break the loop.
    4. Return this reference. (code)
42. [ARRAY] https://leetcode.com/problems/longest-continuous-increasing-subsequence/description/
    1. Longest Continuous Increasing Sub Sequence
    2. For each element of the array, I go forward as long as the previous element (prev) is lesser than the next one. If it isn't then, I append the length of the increasing sequence to a list.
    3. Sometimes, the list is already sorted in ascending order, so prev is never more than the next. And hence, no value is appended to my list. That's why, I use a flag variable to check if a length was ever appended to my list. If it wasn't then when I come out of the j or nested loop, I append the length anyways. Each time the length is reset.
    4. This is O(n^2) and pretty slow, but it gets the job done!
43. [STRINGS] https://leetcode.com/problems/find-the-difference/?envType=study-plan-v2&envId=programming-skills
    1. two strings, s and t. t has one extra character that s does not have.
    2. Goal: Print out this extra character.
    3. Solution: Loop through t. Check if that character exists in s. If it does, remove it from s. The first character you find that does not exists in s, is your correct answer.
       1. Note: removal from s is important due to the case of s='a' and t='aa'

1. [ARRAY] https://leetcode.com/problems/contains-duplicate/description/
   1. This is the first problem on Neetcode's roadmap.
   2. Return True if the Array has duplicate elements, else return False.
   3. Initially I tried to keep a list called app. As I iterated through the array I checked if that particular element was already in app or not. If it was already present in app I returned True as that meant I'd seen the number before and the array did indeed contain duplicate elements. If the number wasn't present in app I appended it to app and continued with life.
   4. However doing the above led to a Timeout error. So I asked GPT to guide me to the right solution using the Socratic Method and it asked me to find a more efficient data structure for the app list which is a Set. So I changed app = [] to app = set() and found that I solved the problem!
   5. Solution here
   6. NeetCode's solution: YouTube, Text
2. [ARRAY] https://leetcode.com/problems/valid-anagram/
   1. Given two strings s and t check if they are anagrams.
   2. I created a list of all the letters in t.
   3. For each character of s if it was present in t_chars I removed that from the t_chars list.
   4. At the end of the loop, if len(t_chars)==0 I returned True, else I returned False.

I know I'm flipping and flopping too much, but a friend of mine recently sent this DSAGUIDE.PDF to me and it's a really well written document that teaches the reader the fundamentals of Data Structures and Algorithms. I like the PDF and hence I'll be using it in my journey from now on.

1. [ARRAY] LeetCoode 771: Jewels and Stones (https://leetcode.com/problems/jewels-and-stones/description/)
   1. So you have two strings, jewels and stones. You have to output the number of jewels in the stones string. Each character in the jewels string is a different type of jewel. And each character in the stones string is a type of stone. Now some stones may be jewels while others aren't jewels.
   2. This is a simple problem, for each character in jewels run a loop through the entire stones string. If a character in stones matches the character in jewels, increment the total_jewels counter by one.
   3. Solution here.

1. https://codeforces.com/problemset/problem/4/A [Watermelon] (15 Apr 2025)
   1. Essentially check if a number is even and greater than 2 so that it can be split into two even numbers.
2. https://codeforces.com/problemset/problem/71/A [Way Too Long Words]
   1. Essentially get the length of a string and if the length of the string is more than 10 then replace it with
3. https://codeforces.com/problemset/problem/231/A [Team]
   1. Count the number of 1's in a bit string. Count if it's more than 2.
4. https://codeforces.com/problemset/problem/282/A [Bit++]
   1. See if the string '++' is in the string and update a variable accordingly.

1. In Pandas, to modify the columns .astype('int') or something is used in general.
2. In Pandas, to rename a column, you have to pass a dictionary of the {'old_col_name': 'new_col_name'} to the .rename() function.
3. In SQL, to use the WHERE clause these two links might be helpful to refresh your memory of the Syntax (here, here)
4. If you want to get the unique values from a column in SQL use the DISTINCT keyword. For example, SELECT DISTINCT column_name FROM table_name; (here)
5. To read numbers in Bash you use read command. For example, read a will read a number and store it in the variable a.
6. To do arithmetic expressions in Bash using the values of variables use the $(()) sign. For example, echo $((a+b)) will print the sum of a and b.
7. In Pandas if you have a column say email in a DataFrame, say, customers and you want to get the drop the rows where a duplicate value occurs then use customers.drop_duplicates(subset=['email']). This will drop the rows where the email column has duplicate values. (Nice article)
8. sorted() in Python returns a list whereas .sort() just does the sort inplace on the list. So, a=sorted(list) makes sense but b=l.sort() will yield None for b.

Starting to maintain a table from now on.