树状数组求逆序数

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#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 500005
#define ll long long
using namespace std;
ll c[maxn << 2], s[maxn << 2], a[maxn << 2], b[maxn << 2];
ll n;

int lowbit(int x){return x & (-x);}

void update(ll i,int x){
    while(i <= n){
        c[i] += x;
        i += lowbit(i);
    }
}

ll Sum(ll x){
    ll sum = 0;
    while(x > 0){
        sum += c[x];
        x -= lowbit(x);
    }
    return sum;
}

int main()
{
    while(~scanf("%lld",&n)){
      if(n == 0) break;
      memset(c, 0, sizeof(c));
      ll ans = 0;
      int m;
      for(int i=1;i<=n;i++){
          scanf("%lld",&a[i]);
          a[i] += 1000000000;
          b[i] = a[i];
      }
      sort(b + 1, b + n + 1);
      int len = unique(b + 1, b + n + 1) - b - 1;
      for(int i=1;i<=n;i++){
          a[i] = lower_bound(b + 1, b + len + 1, a[i]) - b;
          update(a[i], 1);
          ans += (ll)i - Sum(a[i]);
      }
      printf("%lld\n",ans);
  }
  return 0;
}