逆序数，结合归并排序。

之前一直用树状数组写的，今天发现归并排序也很好写。

https://www.nowcoder.com/practice/96bd6684e04a44eb80e6a68efc0ec6c5?tab=answerKey

class Solution {
public:int a[200005] = {0}, tmp[200005] = {0};int sort(int left, int mid, int right) {int p1 = left, p2 = mid+1, k = left, ans = 0;while(p1 <= mid && p2 <= right) {if(a[p1] <= a[p2]) tmp[k++] = a[p1++];else {ans += mid-p1+1; tmp[k++] = a[p2++];}//ans += p2-k也可以，加上左侧还剩余的}while(p1 <= mid) tmp[k++] = a[p1++];while(p2 <= right) tmp[k++] = a[p2++];for(int i = left; i<=right; i++) a[i] = tmp[i];return ans;}int mergesort(int l, int r) {int m, ret = 0;if(l<r) {m = (l+r)/2;ret += mergesort(l, m);ret %= 1000000007;ret += mergesort(m+1, r);ret %= 1000000007;ret += sort(l,m,r);ret %= 1000000007;}return ret;}int InversePairs(vector<int> data) {for(int i = 0; i<data.size(); i++) a[i] = data[i];int res = mergesort(0, data.size()-1);return res;}
};