POJ2299

POJ2299 只要理清逆序数的思想，再套上归并算法的模板再注意点 long long sum。不能用int就ok了。

Source Code

Problem: 2299		User: 64162451
Memory: 3820K		Time: 1266MS
Language: C++		Result: Accepted

• Source Code

#include<iostream>
#include<fstream>
using namespace std;
const int N=500005;
int a[N];
int t[N];
long long sum;
void copy(int *dest,int *src,int b,int e)
{
	while(b<=e)
	{
		dest[b]=src[b];
		b++;
	}
}
void merge(int * a,int b,int m, int e)
{
	int i=b;
	int j=m+1;
	int k=b;
	while((i<=m)&&(j<=e))
	{
		if(a[i]<=a[j])
			t[k++]=a[i++];

		else
		{
			t[k++]=a[j++];
			sum+=(m-i+1);
		}
	}
	while(i<=m)
	{
		t[k++]=a[i++];
	}
	while(j<=e)
	{
		t[k++]=a[j++];
	}
	copy(a,t,b,e);
}
void mergesort(int * a,int i,int j)
{
		if(i>=j)return;
		int m=(i+j)/2;
		mergesort(a,i,m);
		mergesort(a,m+1,j);
		merge(a,i,m,j);
	
}
int main()
{
	//fstream cin("C:\\Users\\wuyanyisb\\Desktop\\1.txt");
	int n;
	while(cin>>n&&n!=0)
	{
		sum=0;
		for(int i=0;i<n;i++)
			cin>>a[i];
		mergesort(a,0,n-1);
		cout<<sum<<endl;
	}
	system("pause");
	return 0;
}