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For an integer array
nums, an inverse pair is a pair of integers
[i, j] where
0 <= i < j < nums.length and
nums[i] > nums[j].
Given two integers n and k, return the number of different arrays consist of numbers from
n such that there are exactly
k inverse pairs. Since the answer can be huge, return it modulo
109 + 7.
Input: n = 3, k = 0 Output: 1 Explanation: Only the array [1,2,3] which consists of numbers from 1 to 3 has exactly 0 inverse pairs.
Input: n = 3, k = 1 Output: 2 Explanation: The array [1,3,2] and [2,1,3] have exactly 1 inverse pair.
1 <= n <= 1000
0 <= k <= 1000