HDU 4911 Inversion

题目链接:http://acm.hdu.edu.cn/showproblem.php?pid=4911


题意:一个含有n个元素的序列,最多可以交换相邻元素k次,求交换后的最小的逆序对数量。


思路:如果存在逆序对,我们肯定有办法通过一次交换来减少一对逆序对。直接用树状数组求一下逆序对即可,max(0,ans-k).


#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

#define rep(i,j,k) for (int i=j;i<=k;i++)
#define Rrep(i,j,k) for (int i=j;i>=k;i--)

#define Clean(x,y) memset(x,y,sizeof(x))
#define LL long long
#define ULL unsigned long long
#define inf 0x7fffffff
#define mod 100000007
const int maxn = 100009;
int n,k;

struct node
{
    int pos,num;
}a[maxn];
int b[maxn];
int N;
int sum[maxn];
bool cmp(node x,node y)
{
    return x.num < y.num;
}
int query( int x )
{
    int ans = 0;
    while( x > 0 )
    {
        ans += sum[x];
        x -= x & (-x);
    }
    return ans;
}
void add( int x )
{
    while( x <= N )
    {
        sum[x]++;
        x+= x & (-x);
    }
}
int main()
{
    while( ~scanf("%d%d",&n,&k) )
    {
        rep(i,1,n)
        {
            scanf("%d",&a[i].num);
            a[i].pos = i;
        }
        sort( a+1,a+1+n,cmp );
        N = 0;
        int last = a[1].num-1;
        rep(i,1,n)
        {
            if ( last != a[i].num )
            {
                N++;
                last = a[i].num;
            }
            b[ a[i].pos ] = N;
        }
        Clean(sum,0);
        LL ans = 0;
        Rrep(i,n,1)
        {
            ans += query( b[i] - 1 );
            add( b[i] );
        }
        printf("%I64d\n",max((LL)0,ans-k));
    }
    return 0;
}


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