- C. Prepend and Append: https://mirror.codeforces.com/problemset/problem/1791/C
- D. Black and White Stripe: https://mirror.codeforces.com/problemset/problem/1690/D
- D. 1D Eraser: https://mirror.codeforces.com/contest/1873/problem/D
- B. Long Long: https://mirror.codeforces.com/problemset/problem/1843/B
