归并排序求逆序数，然后ans-k与0取一个最大值就可以了。

也可以用树状数组做，比赛的时候可能姿势不对，树状数组wa了、、

Inversion

3 1
2 2 1
3 0
2 2 1

1
2

#include <algorithm>
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <iomanip>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>
#include <stack>
#include <map>
#include <set>
#define eps 1e-12
///#define M 1000100
#define LL __int64
///#define LL long long
///#define INF 0x7ffffff
#define INF 0x3f3f3f3f
#define PI 3.1415926535898
#define zero(x) ((fabs(x)<eps)?0:x)

using namespace std;


const int maxn = 10010000;

int num[maxn];
LL ans;
int tmp[maxn];

void Sell(int l, int mid, int r)
{
    int i = l;
    int j = mid+1;
    int k = 0;
    while(i <= mid && j <= r)
    {
        if(num[i] > num[j])
        {
            tmp[k++] = num[j++];
            ans += mid-i+1;
        }
        else tmp[k++] = num[i++];
    }
    while(i <= mid)
        tmp[k++] = num[i++];
    while(j <= r)
        tmp[k++] = num[j++];
    for(int i = 0; i < k; i++)
        num[i+l] = tmp[i];
}

void Sort(int l, int r)
{
    if(l < r)
    {
        int mid = (l+r)/2;
        Sort(l, mid);
        Sort(mid+1, r);
        Sell(l, mid, r);
    }
}
int main()
{
    int n, k;
    while(cin >>n>>k)
    {
        ans = 0;
        for(int  i = 0;i < n; i++) scanf("%d",&num[i]);
        Sort(0, n-1);
        cout<<max(0LL, ans-k)<<endl;
    }
    return 0;
}

HDU 4911 Inversion(归并排序求逆序数),布布扣,bubuko.com

HDU 4911 Inversion(归并排序求逆序数)

原文：http://blog.csdn.net/xu12110501127/article/details/38396323