超级快排：Ultra-QuickSort

题意：

求完成排序后最少交换了几次，这道题，如果有了解，呢么就会知道是并归排序，把并归排序写下来，在并归排序中，求交换次数即可

AC代码：

#include<cstdio>
using namespace std;
//并归排序
#define ll long long
ll ans;
void Merge(int A[],int low,int mid,int high)
{
    int *B=new int [high-low+1];//申请一个辅助数组
    int i=low,j=mid+1,k=0;
    while(i<=mid&&j<=high)
    {

        if(A[i]<=A[j])
        {
            B[k++]=A[i++];

        }
        else
        {
            B[k++]=A[j++];
            ans+=(mid+1-i);//求交换了几次,因为并归排序本身就是最优
        }
    }
    while(i<=mid)
        B[k++]=A[i++];
    while(j<=high)
        B[k++]=A[j++];
    for(i=low,k=0; i<=high; i++)
    {
        A[i]=B[k++];

    }
    delete []B;
}
void MergeSort(int A[],int low,int high)
{
    if(low<high)
    {
        int mid=(low+high)/2;
        MergeSort(A,low,mid);
        MergeSort(A,mid+1,high);
        Merge(A,low,mid,high);



    }
}
int main()
{


    int a[500005],n;
    while(~scanf("%d",&n)&&n)
    {
        //memset(a,0,sizeof(a));
        ans=0;
        for(int i=0; i<n; i++)
        {
            scanf("%d",&a[i]);
        }
        MergeSort(a,0,n-1);
//for(int i=0;i<n;i++)
        //cout<<a[i]<<' ';
        printf("%lld\n",ans);



    }







}