POJ 2299

 1 #include<iostream>
 2 #include<stdio.h>
 3 #define MAXN 500000  
 4 using namespace std;
 5 
 6 int a[MAXN];
 7 int c[MAXN];
 8 long long n;
 9 void Merge(int le, int mid, int rh);
10 void MergeSort(int le, int rh);
11 int main()
12 {
13     //freopen("acm.acm","r",stdin);
14     int num;
15     int i;
16     while(cin>>num && num)
17     {
18         n = 0;
19         for(i = 0; i < num; ++ i)
20             cin>>a[i];
21         MergeSort(0,num-1);
22         //for(i = 0; i < num; ++ i)
23         //    cout<<a[i]<<" ";
24         //cout<<endl;
25         cout<<n<<endl;
26     }
27 }
28 
29 void MergeSort(int le, int rh){
30   
31      
32      if (rh>le)
33      {
34          int mid = (le+rh) >> 1;
35          MergeSort(le,mid);
36          MergeSort(mid + 1,rh);
37          Merge(le, mid, rh);
38      }
39  }
40  
41  void Merge(int le, int mid, int rh)
42  {
43          int i, j,p = 1;
44          for (i = le, j = mid+1; i <= mid && j <= rh; ) 
45          {
46              if (a[j] < a[i])
47              {
48                  c[p++] = a[j++];
49                  n += mid - i + 1;    
50              }
51              else   c[p++] = a[i++];
52          }
53         while ( j <= rh )
54                  c[p++] = a[j++];
55         while( i <= mid )  
56                  c[p++] = a[i++];
57  
58          for (i = le; i <= rh; ++i)        
59          {
60              a[i] = c[i - le + 1];
61          }
62  }