POJ 2299 Ultra-QuickSort(归并排序求逆序数)

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#include<iostream>
#include<cstdio>
#include<vector>
using namespace std;
int n;
long long ans;
long long v[500005];
long long temp[500005];
void merge(int low, int mild, int hight){
	int i = low, j = mild + 1, k = low;
	while(i <= mild && j <= hight){
		if(v[i] <= v[j])	temp[k++] = v[i++];
		else{
			ans += j - k;
			temp[k++] = v[j++];
		}
	}
	while(i <= mild)	temp[k++] = v[i++];
	while(j <= hight)	temp[k++] = v[j++];
	for(i = low; i <= hight; ++i)	v[i] = temp[i];
}
void mergeSort(int start, int end){
	if(start < end){
		int mild = (start + end)>>1;
		mergeSort(start, mild);
		mergeSort(mild + 1, end);
		merge(start, mild, end);
	}
}
int main(){
	while(~scanf("%d", &n) && n){
		ans = 0;
		for(int i = 0; i < n; ++i)	scanf("%lld", &v[i]);
		mergeSort(0, n - 1);
		printf("%lld\n", ans);
	}
	return 0;
}