Absurd Problems

C. Fillomino 2 
Problem - C - Codeforces

Not easy to construct the solution, but there is a nice way to solve it. 

Construction 2: Construct the region for (1,1),(2,2),…,(n,n)$(1,1),(2,2),\dots ,(n,n)$ in order. When starting at (i,i)$(i,i)$, we walk from (i,i)$(i,i)$ for x$x$ steps, where x$x$ is the number written on (i,i)$(i,i)$. For each step, if the cell to the left of your current cell is empty, we go to that cell and write x$x$ on it. Otherwise we go down and write x$x$ there.

C. Row GCD

Problem - C - Codeforces

GCD(a1+bj,…,an+bj)=GCD(a1+bj,a2−a1,…,an−a1).GCD(a1+bj,…,an+bj)=GCD(a1+bj,a2−a1,…,an−a1)

 GCD(a1+bj,…,an+bj)=GCD(a1+bj,a2−a1,…,an−a1)

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C. DMCA

Problem - P - Codeforces

digit root - keep adding number's digits until it gets less than 10