2016暑假集训训练2 H题 Frosh Week

求逆序数的傻逼题。

用归并排序或者树状数组或者线段树

归并排序代码：

# include<stdio.h>
# define MAXN 500100

int a[MAXN],b[MAXN];
long long ans;

void merge(int l, int r)
{
	int k, p, q;
    if(l == r) return;
    int mid= (l+r)>>1;
    merge(l,mid);
    merge(mid+1,r);
    p = l; q = mid+1; k = l;
    while(p<=mid || q<=r) {
        if(p>mid || (q<=r && a[p]>a[q])) {
            b[k++]=a[q++];
            ans+=mid-p+1;
        }
        else
            b[k++]=a[p++];
    }
    for(k=l; k<=r; k++) a[k] = b[k];
    return ;

}

int main()
{
    int n;
    while(~scanf("%d",&n)) {
		ans = 0;
        for(int i=0;i<n;i++)
            scanf("%d",a+i);
        merge(0,n-1);
        printf("%I64d\n",ans);
    }
    return 0;
}