你的题目我都会看。 把你的排列组合第二题推广一下。 N 个茶杯和编号为1,2, ... N 的杯盖 ,将 N 个杯改盖在 N个茶杯上,至少有2个杯盖与茶杯的编号相同的放法有多少种? |
A popular related problem is: How many ways are there to make sure that none of the lid covers the right cup. If you can solve that problem, say it is f(n), then the answer to QFT's problem is: n!-f(n)-n f(n-1) |
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