629. K Inverse Pairs Array

Given two integers n and k, find how many different arrays consist of numbers from 1 to n such that there are exactly k inverse pairs.

We define an inverse pair as following: For ith and jth element in the array, if i < j and a[i] > a[j] then it’s an inverse pair; Otherwise, it’s not.

Since the answer may be very large, the answer should be 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 pair.

Example 2:

Input: n = 3, k = 1

Output: 2

Explanation:

The array [1,3,2] and [2,1,3] have exactly 1 inverse pair.

Note:

The integer n is in the range [1, 1000] and k is in the range [0, 1000].

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https://leetcode.com/problems/k-inverse-pairs-array/description/

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