POJ 2299（归并求逆序对）

#include <cstdio>
#include <cstring>
using namespace std;
typedef long long ll;
const int N = 5e5+10;
int n;
ll cnt;
int s[N], a[N], tmp[N];
inline void merge_sort(int a[], int l, int r){
    if(l == r) return;
    int mid = (l + r) >> 1;
    merge_sort(a, l, mid);
    merge_sort(a, mid + 1, r);
    int i = l, j = mid + 1, k = 0;
    while(i <= mid && j <= r){
        if(a[i] <= a[j]) tmp[k++] = a[i++];
        else tmp[k++] = a[j++], cnt += (ll)mid - i + 1;
    }
    while(i <= mid) tmp[k++] = a[i++];
    while(j <= r) tmp[k++] = a[j++];
    for(i = 0; i < k; i++){
        a[i + l] = tmp[i];
    }
}
int main(){
#ifdef ONLINE_JUDGE
#else
    freopen("in.txt", "r", stdin);
#endif //ONLINE_JUDGE
    while(scanf("%d", &n), n){
        cnt = 0;
        for(int i = 1; i <= n; i++){
            scanf("%d", &s[i]);
            a[i] = s[i];
        }
        merge_sort(a, 1, n);
        printf("%lld
", cnt);
    }
    return 0;
}