【洛谷】P1908逆序对

这就是归并排序求逆序对啦！

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
const int MAXN=40000+10;
int a[MAXN],ans;
int tmp[MAXN];
void merge(int l,int r)
{
    int mid=((l+r)>>1);
    if(l==r)return;
    merge(l,mid);
    merge(mid+1,r);
    int i,j;
    int k=l-1;
    i=l;j=mid+1;
    while(i<=mid&&j<=r)
    {
        if(a[i]>a[j])
        {
            k++;
            tmp[k]=a[j++];
            ans+=(mid-i)+1;
        }
        else
        {
            k++;
            tmp[k]=a[i++];
        }
    }
    while(i<=mid)tmp[++k]=a[i++];
    while(j<=r)tmp[++k]=a[j++];
    for(i=l;i<=r;i++)
        a[i]=tmp[i];
}
int main()
{
    int t,i,j;
    int n;
//    freopen("Mogic.in","r",stdin);
//    freopen("Mogic.out","w",stdout);
    memset(a,0,sizeof(a));
    scanf("%d",&n);
    for(j=1;j<=n;j++)
        scanf("%d",&a[j]);
    ans=0;
    merge(1,n);
    printf("%d\n",ans);
    return 0;
}