Here 7bitmask problems for you :
https://mirror.codeforces.com/problemset/problem/2035/C
https://mirror.codeforces.com/problemset/problem/1918/C
https://mirror.codeforces.com/problemset/problem/2020/C
https://mirror.codeforces.com/problemset/problem/1988/C
https://mirror.codeforces.com/problemset/problem/1981/B
https://mirror.codeforces.com/problemset/problem/1926/E
https://mirror.codeforces.com/problemset/problem/1926/D
I solved those it was fun..
NOTE : These problems are from recent contests.
