Add missing practice exercises

[marcelweikum]

Hey,

This is a list of all missing (not deprecated) exercises in the C++ track.

Let me know which one you’re picking up, and then go ahead and start the PR. I’ll maintain and update the list accordingly.

• [ ] baffling-birthdays (@marcelweikum) #977
• [ ] book-store (@marcelweikum) #974
• [ ] bottle-song (@marcelweikum) #979
• [ ] bowling (@marcelweikum) #982
• [ ] change (@marcelweikum) #983
• [ ] connect (@marcelweikum) #984
• [ ] custom-set
• [ ] dominoes
• [ ] dot-dsl — (canonical-data.json missing)
• [ ] error-handling — (canonical-data.json missing)
• [ ] flatten-array
• [ ] forth
• [ ] game-of-life
• [ ] go-counting
• [ ] grep
• [ ] hangman — (canonical-data.json missing)
• [ ] house
• [ ] intergalactic-transmission
• [ ] killer-sudoku-helper
• [ ] ledger
• [ ] lens-person — (canonical-data.json missing)
• [ ] markdown
• [ ] matrix
• [ ] micro-blog
• [ ] ocr-numbers
• [ ] paasio — (canonical-data.json missing)
• [ ] palindrome-products
• [ ] piecing-it-together
• [ ] poker
• [ ] pov
• [ ] proverb
• [ ] pythagorean-triplet
• [ ] rational-numbers
• [ ] react
• [ ] rectangles
• [ ] relative-distance
• [ ] resistor-color-trio
• [ ] rest-api
• [ ] saddle-points
• [ ] satellite
• [ ] sgf-parsing
• [ ] simple-cipher
• [ ] split-second-stopwatch
• [ ] square-root
• [ ] state-of-tic-tac-toe
• [ ] strain
• [ ] swift-scheduling
• [ ] tournament
• [ ] transpose
• [ ] tree-building — (canonical-data.json missing)
• [x] twelve-days (@alienx5499) #992
• [ ] variable-length-quantity
• [ ] word-search
• [ ] wordy
• [ ] zipper

Not sure how to get started? This might help: Practice Exercises

Happy coding!