Added 6 Leetcode problems.

[Adi1222]

Problems added:

1. Combination Sum 2 https://leetcode.com/problems/combination-sum-ii/
2. Combination Sum https://leetcode.com/problems/combination-sum/
3. Longest Arithmetic Subsequence https://leetcode.com/problems/longest-arithmetic-subsequence/
4. Coin Change https://leetcode.com/problems/coin-change/
5. Maximum Earnings From Taxi https://leetcode.com/problems/maximum-earnings-from-taxi/
6. Maximum Number of Events That Can Be Attended II https://leetcode.com/problems/maximum-number-of-events-that-can-be-attended-ii/

Please do review my request and merge it. Thanks !