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;
}