pku 2299 Ultra-QuickSort(求逆序，归并排序)

利用归并排序求数组总逆序。

#include <iostream>using namespace std;int original[500005],temp[500005];__int64 reverse;void Merge(int first,int mid,int last){int a=first,b=mid+1,p=first;while((a<=mid)&&(b<=last)){if(original[a]<original[b]){temp[p++]=original[a++];}else{temp[p++]=original[b++];reverse+=mid-a+1;//统计逆序数}}while(a<=mid) temp[p++]=original[a++];while(b<=last) temp[p++]=original[b++];for(int i=first;i<=last;i++){original[i]=temp[i];}}void MergeSort(int first,int end)//第一个元素和最后一个元素的下标{int mid=(first+end)/2;if(first<end){MergeSort(first,mid);MergeSort(mid+1,end);Merge(first,mid,end);}}int main(){int n;while(scanf("%d",&n)&&n){reverse=0;for(int i=0;i<n;i++){scanf("%d",&original[i]);}MergeSort(0,n-1);printf("%I64d/n",reverse);}return 0;}