POJ 2299

求逆序对，数据量大，离散化，当然，这是树状数组的做法。

#define FOPEN freopen("d:\\1.txt","r",stdin)
#define lowbit(x) x & -x
struct sta
{
    int va,idx;
    int sta;
};
sta s[MAXN];
int n,a;
int num[MAXN];
int cmp(const void *a,const void *b)
{
    struct sta *c = (sta *)a;
    struct sta *d = (sta *)b;
    return d->va - c->va;
}
int cmp1(const void *a,const void *b)
{
    struct sta *c = (sta *)a;
    struct sta *d = (sta *)b;
    return c->sta - d->sta;
}
void add(int x,int va)
{
    while(x <= n+1)
    {
        num[x] += va;
        x += lowbit(x);
    }
}
int qur(int x)
{
    int s=0;
    while(x)
    {
        s += num[x];
        x -= lowbit(x);
    }
    return s;
}
int main()
{
    FOPEN;
    while(~SCF(n)&&n)
    {
        SET(num,0);
        F(i,n) SCF(s[i].va),s[i].sta = i;
        qsort(s,n,sizeof(s[0]),cmp);
        F(i,n) s[i].idx = i+1;
        qsort(s,n,sizeof(s[0]),cmp1);
        long long sum = 0;
        F(i,n)
        {
            sum += qur(s[i].idx);
            add(s[i].idx,1);
        }
        printf("%lld\n",sum);
    }
}