#A. Devu, the Singer and Churu, the Joker
(略)
#B. Devu, the Dumb Guy
(略),至于为什么是对的我想不需要证明……
#C. Devu and Partitioning of the Array
(稍后补)
#D. Devu and his Brother
将数组$a$升序排序,将$b$降序排序
答案就是\(\sum b_i-a_i\)(\(b_i>a_i\))
为什么是对的?
设$a$数组最小的一项不小于\(x\),\(b\)数组最大的一项不大于\(x\)
显然,对于每一组\((a_i,b_i)\)满足\(a_i<b_i\),对答案的贡献为\((x-a_i)+(b_i-x)\)
后面就不言而喻了……
#E. Devu and Birthday Celebration
首先,用插板法求出\(n\)划分\(f\)份有序的方案数\(\left(\begin{array}{c}n-1\\\\f-1\end{array}\right)\)
然后,用容斥原理加DP即可