51Nod 1019 逆序数 (归并排序)

 #include <iostream>
 #include <cstring>

 using namespace std;
 const int maxn = ;
 int a[maxn];
 int res[maxn];
 int ans;

 //归并排序
 void merge(int l, int r){
     //cout<<l<<"  "<<r<<endl;
     int mid = (l + r) >> ;
     int i = l, j = mid + ;
     int cur = l;
     while (i <= mid && j <= r){
         if (a[i] <= a[j])
             res[cur++] = a[i++];
         else{
             res[cur++] = a[j++];
             ans += mid - i + ;        //找到逆序的个数
         }
     }
     while (i <= mid)    res[cur++] = a[i++];
     while (j <= r)    res[cur++] = a[j++];
     //排序好后
     for (int i = l; i <= r; i++)    a[i] = res[i];
 }
 void mer_sort(int l, int r){
     if (l < r){
         int mid = (l + r) >> ;
         mer_sort(l, mid);            //分解
         mer_sort(mid + , r);        //分解
         merge(l, r);                //合并
     }
 }
 int main()
 {
     int n;
     while (cin >> n){
         for (int i = ; i <= n; i++)    cin >> a[i];

         ans = ;
         mer_sort(, n);
         cout << ans << endl;
     }
     return ;
 }