StaffDynamic Programming
K Inverse Pairs Array
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 consisting of numbers from 1 to n such that there are exactly k inverse pairs. Since the answer can be huge, return it modulo 10^9 + 7.
Example 1
Input
n = 3, k = 0Output
1Only the array [1, 2, 3], which consists of numbers from 1 to 3, has exactly 0 inverse pairs.
Example 2
Input
n = 3, k = 1Output
2The arrays [1, 3, 2] and [2, 1, 3] have exactly 1 inverse pair.
Constraints
- 1 <= n <= 1000
- 0 <= k <= 1000