Must-do problems: Time and Space Complexity

Every problem here has an obvious solution that is too slow for the largest input, and a better one that passes. Read the limits, estimate the steps of your first idea, and replace the slow loop. Then state the time and space of your answer.

1. 1.Duplicate order idsEasy
2. 2.Valid ticket checkEasy
3. 3.Missing invoice numberEasy
4. 4.Best-selling productEasy
5. 5.Total of the largest ordersEasy
6. 6.Count pairs with a target totalMedium
7. 7.Range totals of daily salesMedium
8. 8.Customers in both reportsMedium
9. 9.Groups of anagram product codesMedium
10. 10.Marks at or below a scoreMedium
11. 11.Ways to climb the warehouse stairsMedium
12. 12.Longest run of different charactersHard
13. 13.Stretches of days with an exact totalHard
14. 14.Longest run of consecutive order numbersHard
15. 15.Distinct products in every windowHard