POJ_2299_Ultra-QuickSort(归并排序，逆序数)

http://poj.org/problem?id=2299

# include <stdio.h>
# include <string.h>
# include <algorithm>
# include <iostream>
using namespace std;
const int MAXN= 500005;
__int64 a[MAXN],b[MAXN];
__int64 cnt;
void merge_sort(__int64 *a,int x,int y,__int64 *b)
{
    __int64 mid,i,j,k;

    if(y>x+1)
    {
        __int64 p,q;
        mid=(x+y)/2;
        p=x;q=mid;i=x;
        merge_sort(a,x,mid,b);
        merge_sort(a,mid,y,b);
        while(p<mid && q<y)
        {
            if(a[p]<=a[q])
            {
                b[i++]=a[p++];
            }
            else 
            {
                b[i++]=a[q++];
                cnt+=mid-p;
            }
        }
        if(p==mid && q!=y)
            while(q!=y)
                b[i++]=a[q++];
        else if(p!=mid && q==y)
            while(p!=mid)
                b[i++]=a[p++];
    /*    另一种写法。
        while(p<mid || q <y)
        {
        if(q>=y || (p<mid && a[p]<=a[q]))
            t[i++]=a[p++];
        else 
        {
            t[i++]=a[q++];
            cnt+=mid-p;
        }
    }*/
        for(i=x;i<y;i++)
            a[i]=b[i];
    }
}
int main()
{
    int n,i,j;
    while(scanf("%d",&n)!=EOF)
    {
        if(n==0)
            break;
        for(i=0;i<n;i++)
        {
            scanf("%I64d",&a[i]);
            b[i]=a[i];
        }
        cnt=0;
        merge_sort(a,0,n,b);
        printf("%I64d\n",cnt);
    }
    return 0;
}