HDU 1394 Minimum Inversion Number

#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;

const int INF=0x3f3f3f3f;
const int N=5010;

struct node
{
    int left, right, num;
}no[4*N];
int Min;

void Bulid(int left, int right, int root)
{
    int mid;

    no[root].left = left;
    no[root].right = right;
    no[root].num = 0; 

    if (left == right) return ;

    mid = (left+right)/2;

    Bulid(left, mid, root*2);
    Bulid(mid+1, right, root*2+1);
}

void Insert(int p, int root) ///插入数据时就可以统计逆序数的个数
{
    int mid;

    no[root].num++;

    if (no[root].left == no[root].right) return ;

    mid = (no[root].left+no[root].right)/2;

    if (p <= mid)
    {
        Min += no[root*2+1].num;
        Insert(p, root*2);
    }
    else Insert(p, root*2+1);
}

int main ()
{
    int i, n, p[N], ans;

    while (scanf("%d", &n) != EOF)
    {
        Bulid(0, n-1, 1);
        Min = 0;

        for (i = 1; i <= n; i++)
        {
            scanf("%d", &p[i]);
            Insert(p[i], 1);
        }

        ans = Min;

        for (i = 1; i <= n; i++)
        {
            Min += (n-1-p[i]) - p[i]; ///首位移到最后一位时，逆序数加上那些比它大的数的个数，减去那些比它小的数的个数（因为该序列的元素都是0~n-1）
            ans = min(ans, Min);
        }

        printf("%d\n", ans);
    }

    return 0;
}