树状数组求逆序数

Posted 2021-02-16 mch5201314

代码

#include<bits/stdc++.h>
using namespace std;
#define MAX_N 100
int bit[MAX_N + 1], n;
long long f;
int a[MAX_N];
int sum(int i){
    int s = 0;
    while(i > 0){
        s += bit[i];
        i -= i & -i;
    }
    return s;
}
 
void add(int i, int x){
    while(i <= n){
        bit[i] += x;
        i += i & -i;
    }
}
void solve()
{
    long long ans = 0;
    for(int j = 0; j < n; ++j){
        ans += j - sum(a[j]);
        add(a[j], 1);
    }
   f=f+ans;
}
int main()
{
    while(~scanf("%d", &n))
   {
    f=0;
    for(int i = 0; i < n; ++i){
        scanf("%d", &a[i]);
    }
    //solve();
    sort(a,a+n);
    do{
        memset(bit,0,sizeof(bit));
      solve();  
    }while(next_permutation(a,a+n));
    cout<<f<<endl;
       }    
    
    return 0;
}