一个初等数论问题的朴素解法。

一个初等数论问题的巧妙解法。

一个贪心算法的正确性证明。

Another typical pair-counting problem.

A typical pair-counting problem.

An interesting problem.

A typical instance of tree DP of merge.

Factorize factorial.

A strict weak ordering on nonempty strings.

一个计数问题。