[算法] doj 1067 归并法求逆序对 Ultra-QuickSort

#include <iostream>
#include <cstdio>
using namespace std;
#define mid(x,y) ((x&y) + ((x^y)>>1))
const int N = 500005;

long long a[N];
long long ans; // 记录逆序对个数
void merge_sort(int l, int r) {
	if(l >= r) return;
	int m = mid(l,r);
	merge_sort(l, m);
	merge_sort(m+1, r);
	long long *arr = new long long[r-l+1];
	int i = l, j = m+1, k = 0;
	while(i <= m && j <= r) {
		if(a[i] <= a[j]) {
			arr[k++] = a[i++];
		}
		else {
			ans += (m - i + 1);
			arr[k++] = a[j++];
		}
	}
	while(i <= m) arr[k++] = a[i++];
	while(j <= r) arr[k++] = a[j++];
	for(int i = l; i <= r; i++) {
		a[i] = arr[i-l];
	}
	delete []arr;
}
int main() {
	int n;
	while(cin >> n, n) {
		for(int i = 0; i < n; i++) {
			cin >> a[i];
		}
		ans = 0;
		merge_sort(0, n-1);
		cout << ans << endl;
	}
	return 0;
}