Must-do problems: Two Pointers

These are the two-pointer problems every placement candidate should be able to solve, from a sorted pair sum to counting triangles. Pick the pattern first: pointers from both ends, read and write, or fast and slow. Trace a small input on paper, then check your program against every test.

1. 1.Two prices that match a budgetEasy
2. 2.Valid palindrome messageEasy
3. 3.Reverse the order listEasy
4. 4.Zeros to the backEasy
5. 5.Unique values of a sorted listEasy
6. 6.Sorted squaresEasy
7. 7.Merge two sorted listsEasy
8. 8.Container with the most waterMedium
9. 9.Pairs under a limitMedium
10. 10.Closest pair of costsMedium
11. 11.Sort the three levelsMedium
12. 12.Triplets that cancel outMedium
13. 13.At most two copiesMedium
14. 14.Middle of a chainMedium
15. 15.Almost a palindromeMedium
16. 16.The repeated ticket numberHard
17. 17.Happy numberHard
18. 18.Closest triple of costsHard
19. 19.Rain between the wallsHard
20. 20.Sticks that make a triangleHard