POJ2299 Ultra-QuickSort

归并排序求逆序对 用long long存结果

#include<cstdio>
using namespace std;
const int maxn = 5e5 + 5;
int a[maxn], b[maxn];
long long cnt;
void Merge(int l,int mid,int r)
{
    int k = l, j = mid+1;
    for(int i = l; i <= r; i++){
        if(j > r || k <= mid && a[k] < a[j]) b[i] = a[k++];
        else b[i] = a[j++], cnt += mid-k+1;
    }
    for(int i = l; i <= r; i++) a[i] = b[i];
}
void mergesort(int x,int y)
{
    if(x < y){
        int mid = (x+y)/2;
        mergesort(x,mid);
        mergesort(mid+1,y);
        Merge(x,mid,y);
    }
}
int main()
{
    int n;
    while(scanf("%d",&n) && n){
        cnt = 0;
        for(int i = 1; i <= n; i++){
            scanf("%d",&a[i]);
        }
        mergesort(1,n);
        printf("%lld\n",cnt);
    }
    return 0;
}