Effect: Given a tree, each step can come from any point of not more than $ k $, denoted by $ f (s, t) $ $ s $ is the number of steps to a minimum of $ t $, seeking $ \ sum \ limits_ { s <t} f (s, t) $
Bear and Tree Jumps CodeForces - 771C (树形dp)
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Origin www.cnblogs.com/uid001/p/11273086.html
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