洛谷P1908 逆序对

题目链接：https://www.luogu.org/problemnew/show/P1908
思路：使用并归排序的思想。
c++代码：

#include 
#include 

using namespace std;

const int maxn=6*1e5;

int a[maxn];
int T[maxn];
long long ans;

void merge_count(int left,int right,int* T) {
   	if(right>left) {
   	   int mid=(right+left)/2;
	   int p=left,q=mid+1,k=left;
	   merge_count(left,mid,T);
	   merge_count(mid+1,right,T);
	   while(p<=mid||q<=right) {
	   	  if(q>right||(p<=mid&&a[p]<=a[q]))T[k++]=a[p++];
	   	  else {
	   	  	   T[k++]=a[q++];
	   	  	   ans+=mid-p+1;
			 }
	   } 	
	   for(k=left;k<=right;k++)
	      a[k]=T[k];
	}
}

int main() {
	int n;
	scanf("%d",&n);
	for(int i=0;i