AcWing 107 超快速排序 (归并排序求逆序对)

https://www.acwing.com/problem/content/109/

归并排序求逆序对模板题
每次合并的时候，对答案的贡献即为在当前左边区间元素被排序之前先被排序的右边区间元素的数量

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<cmath>
#include<stack>
#include<queue>
using namespace std;
typedef long long ll;

const int maxn = 500010;

int n; ll ans;
int a[maxn], b[maxn];

void Merge(int l, int r, int mid){
	int i = l, j = mid + 1, pos = l;
	for(;i <= mid && j <= r;){
		if(a[i] <= a[j]){
			b[pos++] = a[i++];
			ans += j - (mid + 1);
		}else {
			b[pos++] = a[j++];
		}
	}
	while(i <= mid){
		b[pos++] = a[i++];
		ans += r - (mid + 1) + 1;
	}
	while(j <= r) b[pos++] = a[j++];
	
	for(i = l; i <= r; ++i) a[i] = b[i];
}

void Merge_Sort(int l, int r){
	if(l == r){ return; }
	int mid = (l + r) >> 1;
	Merge_Sort(l, mid);
	Merge_Sort(mid+1, r);
	Merge(l,r,mid);
}

ll read(){ ll s=0,f=1; char ch=getchar(); while(ch<'0' || ch>'9'){ if(ch=='-') f=-1; ch=getchar(); } while(ch>='0' && ch<='9'){ s=s*10+ch-'0'; ch=getchar(); } return s*f; }

int main(){
	while(1){
		n = read(); ans = 0;
		if(n == 0) break;
		
		for(int i=1;i<=n;++i) a[i] = read();
		
		Merge_Sort(1,n);
		
		printf("%lld
",ans); 
	}
	return 0;
}