求逆序数#no

今天做了下POJ2299，逆序数的2种方法：


1.归并排序法：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
long long  ans;
int c[500100];
int a[500100];
void merge(int l,int m,int r)
{
    int i = l,j = m+1;
    int cnt = 0;
    while(i<=m && j<=r)
    {
        if(a[i]<a[j]){c[cnt++] = a[i];i++;}
        else
        {
             ans += m-i+1;
             c[cnt++] = a[j];
             j++;
        }
    }
    //cout << ans << endl;
    while(i<=m){c[cnt++] = a[i];i++;}
    while(j<=r){c[cnt++] = a[j];j++;}
    //cout << cnt << endl;
    for(i=l;i<=r;i++)
        a[i] = c[i-l];
}
void mergesort(int l,int r)
{
    if(l<r)
    {
        int m  = (r+l)>>1;
        mergesort(l,m);
        mergesort(m+1,r);
        merge(l,m,r);
    }
}
int main()
{
    //freopen("input.txt","r",stdin);
    int n;
    while(scanf("%d",&n),n)
    {
        ans = 0;
        for(int i=0;i<n;i++)
            scanf("%d",&a[i]);
        mergesort(0,n-1);
        printf("%I64d\n",ans);
    }
    return 0;
}

2.树状数组：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define lowbit(x) (x&-x)
struct node
{
    int val,no;
};
node a[500100];
int aa[500100];
int c[500100];
int n;
void update(int x,int d)
{
    for(int i=x;i<=n;i+=lowbit(i))
        c[i]+=d;
}
int getsum(int x)
{
    int sum = 0;
    for(int i=x;i>0;i-=lowbit(i))
        sum += c[i];
    return sum;
}
bool cmp(const node& n1,const node& n2)
{
    return n1.val < n2.val;
}
int main()
{
    while(scanf("%d",&n),n)
    {
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i].val);
            a[i].no = i;
        }
        sort(a+1,a+n+1,cmp);
        for(int i=1;i<=n;i++)
            aa[a[i].no] = i;
        memset(c,0,sizeof(c));
        long long ans = 0;
        for(int i=1;i<=n;i++)
        {
            update(aa[i],1);
            ans += i-getsum(aa[i]);
        }
        printf("%I64d\n",ans);
    }
    return 0;
}