629. K Inverse Pairs Array

标签：rom   put   array   any   exactly   pair   bsp   hat   different   

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.

629. K Inverse Pairs Array

标签：rom   put   array   any   exactly   pair   bsp   hat   different   

原文地址：http://www.cnblogs.com/jxr041100/p/7947548.html