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+1));
    if(!sort_data) return;
    int i=le,j=mid+1,pt=0;
    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+1;
        }
    }
    while(i<=mid){
        sort_data[pt++]=a[i++];
    }
    while(j<=rt){
        sort_data[pt++]=a[j++];
    }
    int p;
    for(int i=le,p=0;i<=rt;i++,p++){
        a[i]=sort_data[p];
    }
}
void mergeSort(int a[],int st,int ed){
    if(st<ed){
        int mid=(st+ed)/2;
        mergeSort(a,st,mid);
        mergeSort(a,mid+1,ed);
        merge(a,st,mid,ed);
    }
}
int main(){
    int n,num[500010];
    while(scanf("%d",&n),n){
        ans=0;
        for(int i=0;i<n;i++){
            scanf("%d",&num[i]);
        } 
        mergeSort(num,0,n-1);
        printf("%I64d\n",ans);
    }
    return 0;
}