hdu1394 Minimum Inversion Number

http://acm.hdu.edu.cn/showproblem.php?pid=1394 

线段树，单点更新

用线段树优化，求逆序数，O（N＊logN）

#include <stdio.h>

#define lson l, m, root<<1
#define rson m+1, r, root<<1|1

const int maxn = 5010;
int sum[maxn<<2];

void push_up(int root)
{
    sum[root] = sum[root<<1] + sum[root<<1|1];
}

void build(int l, int r, int root)
{
    int m;
    if(l == r)
    {
        sum[root] = 0;
        return;
    }
    m = (l + r) >> 1;
    build(lson);
    build(rson);
    push_up(root);
}

void update(int x, int one, int l, int r, int root)
{
    int m;
    if(l == r)
    {
        sum[root] = one;
        return;
    }
    m = (l + r) >> 1;
    if(x <= m)
    {
        update(x, one, lson);
    }
    if(m+1 <= x)
    {
        update(x, one, rson);
    }
    push_up(root);
}

int query(int L, int R, int l, int r, int root)
{
    int m, result;
    if(L <= l && r <= R)
    {
        return sum[root];
    }
    result = 0;
    m = (l + r) >> 1;
    if(L <= m)
    {
        result += query(L, R, lson);
    }
    if(m+1 <= R)
    {
        result += query(L, R, rson);
    }
    return result;
}

int main()
{
    int n, a[maxn], i, sum, result;
    while(~scanf("%d", &n))
    {
        build(1, n, 1);
        sum = 0;
        for(i=1; i<=n; i++)
        {
            scanf("%d", a+i);
            a[i] ++;
            sum += query(a[i], n, 1, n, 1);
            update(a[i], 1, 1, n, 1);
        }
        //printf("%d\n", sum);
        result = sum;
        for(i=1; i<=n-1; i++)
        {
            //printf("%d + %d == ", sum, n-1-(a[i]-1)*2);
            sum += (n-1-(a[i]-1)*2);
            //printf("%d\n", sum);
            if(sum < result)
            {
                result = sum;
            }
        }
        printf("%d\n", result);
    }
    return 0;
}