Must-do problems: Array and String Techniques

These are the array and string problems that appear again and again in coding rounds, from reversing a list to the longest run without repeats. Read the input, pick the pattern, write the loop, then check every test. Say the time and space cost aloud when you finish.

1. 1.Reverse a listEasy
2. 2.Swap the first and last ordersEasy
3. 3.Move zeros to the endEasy
4. 4.Rotate right by kEasy
5. 5.Palindrome checkEasy
6. 6.Most frequent letterEasy
7. 7.Remove a value in placeEasy
8. 8.Merge two sorted listsMedium
9. 9.Remove duplicates from a sorted listMedium
10. 10.Sort coloursMedium
11. 11.Maximum subarray sumMedium
12. 12.Valid anagramMedium
13. 13.First non-repeating characterMedium
14. 14.Reverse the wordsMedium
15. 15.Best run with its rangeHard
16. 16.Common items of two sorted listsHard
17. 17.Longest palindromic substringHard
18. 18.Best day to buy and sellHard
19. 19.Group the anagramsHard
20. 20.Longest substring without repeatsHard