Minimum Inversion Number

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int MAX = 5555;
struct segtree{
    int left;
    int right;
    int val;
}tt[MAX << 2];
void build(int n, int left ,int right)
{
    tt[n].left = left;
    tt[n].right = right;
    tt[n].val = 0;
    if(left == right) return ;
    int mid = (tt[n].left + tt[n].right) >> 1;
    build(n << 1, left, mid);
    build(n << 1|1, mid + 1, right);
}

int query(int n, int left ,int right)
{
    if(tt[n].left == left && tt[n].right == right)
        return tt[n].val;//根节点
   int mid = (tt[n].left + tt[n].right) >> 1;
   if(right <= mid)
   return query(n << 1, left ,right);
  else if(left > mid)
    return query(n << 1|1, left, right);
  else return query(n << 1,left, mid) + query(n << 1|1, mid + 1,right);
}
void update(int n, int idx)
{
    tt[idx].val++;
    if(tt[idx].left == tt[idx].right) return ;
    int mid = (tt[idx].left + tt[idx].right) >> 1;
    if( n <= mid )
        update(n, idx << 1);
    else update(n , idx << 1|1);
}
int main()
{
    int n,a[MAX], ans ;
    while(~scanf("%d",&n)){
        build(1, 0, n - 1 );
       int sum = 0;
            for(int i = 0; i < n ; i++){
                scanf("%d", &a[i]);
                sum += query(1,a[i],n - 1);
                update(a[i],1);
            }
            ans = sum;
            for(int i = 0; i < n ; i++){
                sum = sum - a[i] + (n - 1 - a[i]);
              ans = min(ans, sum);
            }
    printf("%d\n",ans);
    }
    return 0;
}