[hdu1394]Minimum Inversion Number(树状数组)

Minimum Inversion Number

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 18395    Accepted Submission(s): 11168

Problem Description

The inversion number of a given number sequence a1, a2, ..., an is the number of pairs (ai, aj) that satisfy i < j and ai > aj.

For a given sequence of numbers a1, a2, ..., an, if we move the first m >= 0 numbers to the end of the seqence, we will obtain another sequence. There are totally n such sequences as the following:

a1, a2, ..., an-1, an (where m = 0 - the initial seqence)
a2, a3, ..., an, a1 (where m = 1)
a3, a4, ..., an, a1, a2 (where m = 2)
...
an, a1, a2, ..., an-1 (where m = n-1)

You are asked to write a program to find the minimum inversion number out of the above sequences.

 

Input

The input consists of a number of test cases. Each case consists of two lines: the first line contains a positive integer n (n <= 5000); the next line contains a permutation of the n integers from 0 to n-1.

 

Output

For each case, output the minimum inversion number on a single line.

 

Sample Input

10
1 3 6 9 0 8 5 7 4 2

 

Sample Output

16

 

Author

CHEN, Gaoli

 

Source

ZOJ Monthly, January 2003

 

Recommend

Ignatius.L

 

 

更更更

 

隔壁YNY看到就直接暴力，当然T了(哈哈哈一起笑他)

动动脑子，题目说是一个环

那么每转一次，就相当于把第一个数放到最后面

考虑第一个数对原有答案的贡献是a[1]-1，也就是小于它的个数(数据是1到n的排列)

最后一个数对原答案的贡献相反

那么移动后当前逆序对数就要减去比第一个数小的个数，再加上比它大的数的个数

这样我们求出一次移动后的逆序对数

这时候我们发现下一次移动直接修改答案就好，不需要改动树状数组

推出式子ans+=n-a[i]-(a[i]-1)

 #include<stdio.h>
 #include<stdlib.h>
 #include<string.h>
 #define LL long long
 int bit[]={},n,a[];
 inline LL min(LL a,LL b){
     return a<b?a:b;
 }
 inline int lb(int x){
     return x&(-x);
 }
 inline LL q(int x){
     LL ans=;
     while(x){
         ans+=bit[x];
         x-=lb(x);
     }
     return ans;
 }
 inline int c(int x){
     while(x<=n){
         bit[x]++;
         x+=lb(x);
     }
     return ;
 }
 int main(){
     while(scanf("%d",&n)!=EOF){
         memset(bit,,sizeof(bit));
         LL ans=;
         for(int i=;i<=n;i++){
             scanf("%d",&a[i]);
             a[i]++;
             ans+=q(n)-q(a[i]);
             c(a[i]);
         }
         LL mn=ans;
         mn=min(mn,ans);
         for(int i=;i<=n;i++){
             ans+=n-a[i]-(a[i]-);
             mn=min(mn,ans);
         }
         printf("%lld\n",mn);
     }

     return ;
 }