Hard
Quiz
#629 K Inverse Pairs Array
APPROACH
An inverse pair in an array is a pair of indices (i, j) with i < j and nums[i] > nums[j]. Given integers n and k, count all permutations of [1..n] that contain exactly k inverse pairs. Return the count modulo 109 + 7.
Example 1:
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.
Example 2:
Input: n = 3, k = 1
Output: 2
Explanation: The array [1,3,2] and [2,1,3] have exactly 1 inverse pair.
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What is the optimal approach for this problem?