SGU 180

求逆序数对    归并排序  

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

int a[71010], b[71010];
long long cnt;
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++], cnt += m-p;
        }
        for(i = x; i < y; i++) A[i] = T[i];
    }
}
int main()
{
    int n;
    scanf("%d",&n);
    for(int i = 0; i < n; i++)
        scanf("%d",&a[i]);
    cnt = 0;
    merge_sort(a, 0, n, b);
    printf("%I64d\n",cnt);
    return 0;
}