ds 5.3 逆序对

/***************
Problem from :
Problem describe :
归并算法求逆序对 
data:2016.12.6
****************/

#include<iostream>
#include<cstdlib>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<map>
#include<stack>
#include<queue>
#include<ctime>
#include<cstring>
#include<vector>
#include<string>
#define ll __int64
#define inf 0x3f3f3f3f3f
using namespace std;
const int maxn = 50005;

int num[maxn]={0}, temp[maxn]={0}, ans=0;
void Merge(int l, int r) //以数组temp作为中间点进行合并    
{
	if(l==r) return ;
	int mid = (l+r)>>1;
	int k=l, i=l, j=mid+1;//左半边有序   右半边有序  要将两个有序的合成一个有序的数组 
	while(i<=mid && j<=r) //小的先进temp数组 
	{
		if(num[i]>num[j]) temp[k]=num[j++], ans += mid-i+1;
		else temp[k] = num[i++];
		k++;
	}
	while(i<=mid) temp[k++] = num[i++];
	while(j<=r) temp[k++]=num[j++];
	
	//将temp中有序的值赋给num 
	for(i=l; i<=r; i++) num[i] = temp[i]; 
	return ;
}
void msort(int l, int r)
{
	if(l>=r) return ;
	int mid = (l+r)>>1;
	msort(l, mid);//排左半边 
	msort(mid+1, r);//排右半边 
	Merge(l,r);//左右合并起来 
	return;
}
int main()
{
//  freopen("in.txt","r",stdin);
//  freopen("out.txt","w",stdout);
	int i, n, flag=0;
	scanf("%d", &n);
	for(i=1; i<=n; i++) scanf("%d", &num[i]);
	msort(1, n);
	printf("%d\n", ans);
	return 0;
}