Must-do problems: Prefix Sums and Hashing

These are the prefix sum and hashing problems every placement candidate should be able to solve, from a plain range query to counting blocks in a table. State the brute force and its cost first. Then write the better version and check it against every test.

1. 1.Range sum queriesEasy
2. 2.Running balanceEasy
3. 3.Repeated order idEasy
4. 4.First repeated idEasy
5. 5.Pair that fits a budgetEasy
6. 6.Balance pointEasy
7. 7.Same lettersEasy
8. 8.Group the anagramsMedium
9. 9.Blocks with a given totalMedium
10. 10.Blocks divisible by kMedium
11. 11.Range add updatesMedium
12. 12.Block sums in a tableMedium
13. 13.Longest block with a given totalMedium
14. 14.Pairs with a given sumMedium
15. 15.Longest run of consecutive numbersMedium
16. 16.Longest balanced runHard
17. 17.Busiest momentHard
18. 18.Four lists with total zeroHard
19. 19.Tables with a given totalHard
20. 20.Shortest removal for a clean totalHard