underdogs29's blog

By underdogs29, history, 2 months ago, In English,

Can anyone give me some ideas for the following problem? http://lightoj.com/volume_showproblem.php?problem=1095


2 months ago, # |
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The solution idea comes from dearrangement. At a time from first m integer you have to dearrange (m-k) integer. And also from (m+1)'th to n'th integer you can dearrange them or not. So iterate over (n-m) and for every time suppose i you keep them at their position, so you have (n-k-i) integer which you can dearrange. Add those ways in every iteration. And at last multiply with mCk