Luogp1908 Reverse Order (merge order), P1908

Luogp1908 Reverse Order (merge order), P1908
Description

TOM and JERRY, the cat and CAT, have been competing again recently, but after all they are adults. They no longer like to play the game that you catch up with. Now they like to play statistics. Recently, TOM found something that humans call "backward-order pairs". This is defined as follows: for a given positive integer sequence, reverse Order pairs are the ordered pairs of ai> aj and I <j in the sequence. After knowing this concept, they will first calculate the number of reverse pairs in a given positive integer sequence.

Input/Output Format

Input Format:

 

The first row, n, indicates the number of n in the sequence.

N number in the second row, indicating the given sequence.

 

Output Format:

 

Number of reverse pairs in a given sequence.

 

Input and Output sample input sample #1: Copy

65 4 2 6 3 1

Output example #1: Copy

11

Description

For 50% of data, n ≤ 2500

For 100% of data, n ≤ 40000.

 

 

 1 #include<cstdio> 2 #include<iostream> 3 #include<cmath> 4 using namespace std; 5 const int MAXN=100001; 6 inline int read() 7 { 8     char c=getchar();int x=0,flag=1; 9     while(c<'0'||c>'9')    {if(c=='-')    flag=-1;c=getchar();}10     while(c>='0'&&c<='9')    x=x*10+c-48,c=getchar();return x*flag;11 }12 int n;13 int a[MAXN];14 int tmp[MAXN];15 int ans=0;16 void Sort(int l,int r)17 {18     if(l==r)    return ;19     int mid=l+r>>1;20     Sort(l,mid);Sort(mid+1,r);21     int nowl=l,nowr=mid+1,nowpos=l;22     while(nowl<=mid&&nowr<=r)23     {24         if(a[nowl]<=a[nowr])    tmp[nowpos++]=a[nowl],nowl++;25         else tmp[nowpos++]=a[nowr],nowr++,ans=ans+mid-nowl+1;26     }27     while(nowl<=mid)    tmp[nowpos++]=a[nowl],nowl++;28     while(nowr<=r)        tmp[nowpos++]=a[nowr],nowr++;29     for(int i=l;i<=r;i++)    a[i]=tmp[i];30 }31 int main()32 {33     n=read();34     for(int i=1;i<=n;i++)    a[i]=read();35     Sort(1,n);36     printf("%d",ans);37     return 0;38 }