uva 10810

刘汝佳书上 143  归并排序求逆序数对

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>

using namespace std;
int a[500010],b[500010];
long long ans;
void merge_sort(int *A, int x, int y, int *T)
{
    if(y - x > 1)
    {
        int m = x + (y-x)/2;
        int p = x, q = m, i = x;
        merge_sort(A, x, m, T);
        merge_sort(A, m, y, T);
        while(p < m || q < y)
        {
            if(q >= y || (p < m && A[p] <= A[q]))
                T[i++] = A[p++];
            else
                T[i++] = A[q++], ans += m-p;
        }
        for(int i = x; i < y; i++)
            A[i] = T[i];
    }
}
int main()
{
    int n;
    while(scanf("%d",&n) == 1 && n)
    {
        ans = 0;
        for(int i = 0 ; i < n; i++)
            scanf("%d",&a[i]);
        merge_sort(a, 0, n, b);
        printf("%lld\n",ans);
    }
    return 0;
}