poj 2299/归并排序、逆序数

归并排序

合并两个有序数组

p,mid

1 3 5 6 7 12

q

2 4 6 8 9 11

找到3比2大，则3之后的都比2大，所以2的逆序数就有mid-p+1个

// 2299.cpp : Defines the entry point for the console application.

//

#include <iostream>

using namespace std;

#define MAX 500000

int arr[MAX];

int tmp[MAX];

__int64 sum ;

void merge(int le,int mi,int ri){

	int p = le,q=mi+1,i=0;

	while(p<=mi&&q<=ri){

		if(arr[p]>arr[q])

		{

			tmp[i]=arr[q++];	

			sum+=mi-p+1;

		}

		else tmp[i]=arr[p++];



		i++;

	}

	while(p <= mi)   

         tmp[i++] = arr[p++];   

        while(q <= ri)   

         tmp[i++] = arr[q++];



	memcpy(arr+le,tmp,(ri-le+1)*sizeof(int));

}

void mergeSort(int left,int right){

	int mid;



	if(left<right)

	{

		mid = (right+left)/2;

		mergeSort(left,mid);

		mergeSort(mid+1,right);

		merge(left,mid,right);

	}

}

int main(int argc, char* argv[])

{

	//freopen("i://in.txt","r",stdin);

	int c,i;

	while(scanf("%d",&c)==1&&c){

		sum = 0;

		for( i=0;i<c;i++)

			scanf("%d",arr+i);

		mergeSort(0,c-1);

		printf("%I64d\n",sum);

		

	}

	

	return 0;

}