poj2299 樹狀數組求逆序對 模闆題

Ultra-QuickSort POJ - 2299

題目：https://vjudge.net/problem/POJ-2299

#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<string.h>
#include<math.h>
#include<string>
#include<vector>
#include<stack>
#include<map>
#include<set>
#include<iostream>
#include<cstdio>
#include<cstring>
#define ll long long
using namespace std;
int n;
ll ans;
const int maxn=500010;
int a[maxn];
int c[maxn];
struct node
{
    int id;
    int val;
}no[maxn];

int cmp(node a,node b)
{
    return a.val<b.val;
}
int ask(int x)
{
   int ans=0;
   for(;x;x-=x&(-x))
         ans+=c[x];
   return ans;
}
int add(int x,int y)
{
    for(;x<=n;x+=x&(-x))
    {
        c[x]+=y;
    }
}
int main()
{

    while(~scanf("%d",&n)&&n)
    {
        memset(c,0,sizeof(c));
        memset(a,0,sizeof(a));
        ans=0;
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&no[i].val);
            no[i].id=i;
        }
        sort(no+1,no+1+n,cmp);
        for(int i=1;i<=n;i++)
        {
            a[no[i].id]=i;
        }

        for(int i=n;i;i--)
        {
            ans+=ask(a[i]-1);
            add(a[i],1);
        }
        printf("%lld\n",ans);

    }
}