POJ2299: Ultra-QuickSort-合并排序解决逆序数问题

 #include<iostream>
 #include<malloc.h>
 using namespace std;
 long long ans;
 void merge(int *a,int le,int mid,int rt){
     int *sort_data=(int *)malloc(sizeof(int)*(rt-le+));
     if(!sort_data) return;
     int i=le,j=mid+,pt=;
     while(i<=mid&&j<=rt){
         if(a[i]<=a[j]){
             sort_data[pt++]=a[i++];
         }
         else{//exist swap action
             sort_data[pt++]=a[j++];
             /*
                 once we swap the postion of both the a[i] and a[j],
                 we do change the positon of a[i+1......mid]
                 so as a result, we do change the postion of a[i,i+1,....mid].
             */
             ans+=mid-i+;
         }
     }
     while(i<=mid){
         sort_data[pt++]=a[i++];
     }
     while(j<=rt){
         sort_data[pt++]=a[j++];
     }
     int p;
     for(int i=le,p=;i<=rt;i++,p++){
         a[i]=sort_data[p];
     }
 }
 void mergeSort(int a[],int st,int ed){
     if(st<ed){
         int mid=(st+ed)/;
         mergeSort(a,st,mid);
         mergeSort(a,mid+,ed);
         merge(a,st,mid,ed);
     }
 }
 int main(){
     int n,num[];
     while(scanf("%d",&n),n){
         ans=;
         for(int i=;i<n;i++){
             scanf("%d",&num[i]);
         }
         mergeSort(num,,n-);
         printf("%I64d\n",ans);
     }
     return ;
 }