NCPC 2012 Bread Sorting

逆序对数的应用；

逆序对数的写法有，二分，树状数组，分治；

学习一下；

树状数组版：

代码：

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 using namespace std;
 const int maxn=;
 int a[maxn],b[maxn],c[maxn];
 int n;
 struct point
 {
     int num,index;
     bool operator<(const point& t)const
     {
         return num<t.num;
     }
 }p[maxn];

 int lowbit(int x)
 {
     return x&(-x);
 }

 void updata(int i,int x)
 {
     while(i<=n)
     {
         c[i]+=x;
         i=i+lowbit(i);
     }
 }

 int sum(int x)
 {
     int ans=;
     while(x>)
     {
        ans+=c[x];
        x-=lowbit(x);
     }
     return ans;
 }

 int main()
 {
     int i,j;
     while(scanf("%d",&n)!=EOF)
     {
         memset(c,,sizeof c);
         for(i=;i<=n;i++)
         {
             scanf("%d",&p[i].num);
             p[i].index=i;
         }
         sort(p+,p+n+);
         for(i=;i<=n;i++)
             b[p[i].index]=i;
         int ans1=,ans2=;
         for(i=;i<=n;i++)
         {
             updata(b[i],);
             ans1=ans1+(sum(n)-sum(b[i]));
         }
         memset(c,,sizeof c);
         memset(b,,sizeof b);
         for(i=;i<=n;i++)
         {
             scanf("%d",&p[i].num);
             p[i].index=i;
         }
         sort(p+,p+n+);
         for(i=;i<=n;i++)
             b[p[i].index]=i;
         for(i=;i<=n;i++)
         {
             updata(b[i],);
             ans2=ans2+(sum(n)-sum(b[i]));
         }
         if(ans1%==ans2%) printf("Possible\n");
         else printf("Impossible\n");
     }
     return ;
 }

分治版：

 #include<cstdio>
 #include<algorithm>
 using namespace std;
 const int maxn=;
 int b[maxn],c[maxn];
 int n,cnt;
 struct point
 {
     int num,index;
     bool operator<(const point& t)const
     {
         return num<t.num;
     }
 }p[maxn];

 void merge_sort(int *a,int x,int y,int *t)
 {
     if(y-x>)
     {
         int m=(y+x)/;
         int p=x,q=m,i=x;
         merge_sort(a,x,m,t);
         merge_sort(a,m,y,t);
         while(p<m||q<y)
         {
             if(q>=y||(p<m&&a[p]<=a[q])) t[i++]=a[p++];
             else
             {
                 t[i++]=a[q++];
                 cnt+=m-p;
             }
         }
         for(int i=x; i<y; i++)a[i]=t[i];
     }
 }
 int main()
 {
     int i,j;
     while(scanf("%d",&n)!=EOF)
     {
         for(i=;i<=n;i++)
         {
             scanf("%d",&p[i].num);
             p[i].index=i;
         }
         sort(p+,p+n+);
         for(i=;i<=n;i++)
             b[p[i].index]=i;
         cnt=;
         merge_sort(b,,n+,c);
         int ans1=cnt;

         cnt=;
         for(i=;i<=n;i++)
         {
             scanf("%d",&p[i].num);
             p[i].index=i;
         }
         sort(p+,p+n+);
         for(i=;i<=n;i++)
             b[p[i].index]=i;
         merge_sort(b,,n+,c);
         int ans2=cnt;
         if(ans1%==ans2%) printf("Possible\n");
         else printf("Impossible\n");
     }
     return ;
 }

线段树版的话就不写了，麻烦！