poj 2299 Ultra-QuickSort(归并排序求逆序数)

来源：互联网发布：南京网络教育编辑：程序博客网时间：2024/05/21 07:51

题目分析：求逆序数，，，

思路：归并排序，关键在于统计逆序数，如果arr[i]>arr[j],则，ans+=m-i+1,

代码：

#include<iostream>#include<cstdio>using namespace std;int n;__int64 ans;int arr[1001000],arr1[1001000];void Merge(int s,int m,int t);void MergeSort(int s,int t){if(s==t){arr1[s]=arr[s];}else{int m=(s+t)/2;MergeSort(s,m);MergeSort(m+1,t);        Merge(s,m,t);}}void Merge(int s,int m,int t){   int i=s,j=m+1,k=s;   while(i<=m&&j<=t)   {       if(arr[i]<=arr[j])   {   arr1[k++]=arr[i++];   }   else   {           arr1[k++]=arr[j++];   ans+=m-i+1;//注意这，，，arr[i]>arr[j],则从arr[i]到arr[m]都是逆序数   }   }   if(i<=m)   while(i<=m)   {   arr1[k++]=arr[i++];   //ans++;   }   else   while(j<=t)   arr1[k++]=arr[j++];   for(int p=s;p<=t;p++)   arr[p]=arr1[p];}int main(){while(scanf("%d",&n)!=EOF&&n!=0){for(int i=1;i<=n;i++)scanf("%d",&arr[i]);ans=0;MergeSort(1,n);/*for(int i=1;i<=n;i++)printf("%d   ",arr[i]);printf("\n");*/printf("%I64d\n",ans);}//system("pause");return 0;}