POJ2299: Ultra-QuickSort-合并排序解决逆序数问题

 1 #include<iostream>

 2 #include<malloc.h>

 3 using namespace std;

 4 long long ans;

 5 void merge(int *a,int le,int mid,int rt){

 6     int *sort_data=(int *)malloc(sizeof(int)*(rt-le+1));

 7     if(!sort_data) return;

 8     int i=le,j=mid+1,pt=0;

 9     while(i<=mid&&j<=rt){

10         if(a[i]<=a[j]){

11             sort_data[pt++]=a[i++];

12         }

13         else{//exist swap action

14             sort_data[pt++]=a[j++];

15             /*

16                 once we swap the postion of both the a[i] and a[j],

17                 we do change the positon of a[i+1......mid]

18                 so as a result, we do change the postion of a[i,i+1,....mid].

19             */

20             ans+=mid-i+1;

21         }

22     }

23     while(i<=mid){

24         sort_data[pt++]=a[i++];

25     }

26     while(j<=rt){

27         sort_data[pt++]=a[j++];

28     }

29     int p;

30     for(int i=le,p=0;i<=rt;i++,p++){

31         a[i]=sort_data[p];

32     }

33 }

34 void mergeSort(int a[],int st,int ed){

35     if(st<ed){

36         int mid=(st+ed)/2;

37         mergeSort(a,st,mid);

38         mergeSort(a,mid+1,ed);

39         merge(a,st,mid,ed);

40     }

41 }

42 int main(){

43     int n,num[500010];

44     while(scanf("%d",&n),n){

45         ans=0;

46         for(int i=0;i<n;i++){

47             scanf("%d",&num[i]);

48         } 

49         mergeSort(num,0,n-1);

50         printf("%I64d\n",ans);

51     }

52     return 0;

53 }

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