Number of Smaller permutations
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I'm practicing programming and I found following problem that I don't know how to solve: We have 2 sequences A and B that both have length N. We should find number of different ways to permute sequence A such that it is lexicographically smaller than the sequence B. Sequence (X_1,X_2,...,X_k) is strictly lexicographically smaller then sequence (Y_1,Y_2,...,Y_k), if there exists and index p (1<=p<=k) such that X_p < Y_p and (X_q=Y_q) for all 1 <= q < p A permutation X of A is considered different form another permutation Y of A if there exists an index i (1<=i<=N) such that X_i != Y_i For example A(2,2,3,3) has 6 different permutations 1<=N<=100000 1<=A_i<=200000 1<=B_i<=200000 Output should be answer mod 1000000007 I cant find general f...