51 Nod 1019 逆序数（归并排序）

 1 #include <iostream>
 2 #include <algorithm>
 3 using namespace std;
 4 
 5 const int N=50000+10;
 6 typedef long long LL;
 7 
 8 LL cnt;
 9 int num[N],result[N];
10 
11 void merge(int *num,int *result,int start,int end){
12     int t=start,mid=(start+end)/2;
13     int i=start,j=mid+1;
14     while(i<=mid&&j<=end){
15         if(num[i]<=num[j]) result[t++]=num[i++];
16         else if(num[i]>num[j]){
17             result[t++]=num[j++];
18             cnt+=mid-i+1;
19         }
20     }
21     while(i<=mid) result[t++]=num[i++];
22     while(j<=end) result[t++]=num[j++];
23     for(int i=start;i<=end;i++) //更新 
24     num[i]=result[i];
25 }
26 
27 void merge_sort(int *num,int *result,int start,int end){
28     if(end-start==1){
29         if(num[start]>num[end]){
30             int tmp=num[start];
31             num[start]=num[end];
32             num[end]=tmp;
33             cnt++;
34         }
35         return ;
36     }
37     if(end-start==0) return ;
38     merge_sort(num,result,start,(end+start)/2);
39     merge_sort(num,result,(end+start)/2+1,end);
40     merge(num,result,start,end);
41 }
42 
43 int main(){
44     int n;
45     cin>>n;
46     for(int i=1;i<=n;i++) cin>>num[i];
47     merge_sort(num,result,1,n);
48     cout<<cnt<<endl;
49     return 0;
50 }