Codeforces Round 315 (Div. 1)

A. Primes or Palindromes?

https://codeforces.com/contest/568/problem/A

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Educational Codeforces Round 152 (Rated for Div. 2)

D. Array Painting

https://codeforces.com/contest/1849/problem/D

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最近公共祖先 (LCA, Lowest Common Ancestor)：树中两个节点的最近公共祖先，就是这两个点的公共祖先里面，离根最远的那个。

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Codeforces Round 852 (Div. 2)

D. Moscow Gorillas

https://codeforces.com/problemset/problem/1793/D

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Codeforces Round 887 (Div. 2)

C. Ntarsis' Set

https://codeforces.com/contest/1853/problem/C

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VK Cup 2015 - Round 2 (unofficial online mirror, Div. 1 only)

B. Work Group

https://codeforces.com/problemset/problem/533/B

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