In combinatorial mathematics, a derangement is a permutation of the elements of a set, such that no element appears in its original position.
There‘s originally an array consisting of n integers from 1 to n in ascending order, you need to find the number of derangement it can generate.
Also, since the answer may be very large, you should return the output mod 109 + 7.
Example 1:
Input: 3 Output: 2 Explanation: The original array is [1,2,3]. The two derangements are [2,3,1] and [3,1,2].
Note:n is in the range of [1, 106].
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[LeetCode] Find the Derangement of An Array 找数组的错排
原文：http://www.cnblogs.com/grandyang/p/7210929.html