Backtracking problems

Exploring choices one at a time, undoing ones that don't pan out.

1. Subsets (Medium)
2. Combination Sum (Medium)
3. Permutations (Medium)
4. Subsets II (Medium)
5. Combination Sum II (Medium)
6. Word Search (Medium)
7. Palindrome Partitioning (Medium)
8. Letter Combinations of a Phone Number (Medium)
9. N-Queens (Hard)