上交oj1219 重要的逆序数对

题意：

https://acm.sjtu.edu.cn/OnlineJudge/problem/1219

思路：

在经典的归并排序求逆序数对算法基础上稍作修改。

实现：

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 using namespace std;
 const int MAXN = ;
 int n, a[MAXN], tmp[MAXN];
 long long cnt = ;
 
 void mergeSort(int * a, int left, int right, int * tmp)
 {
     if (left >= right)
         return;
     int mid = (left + right) / ;
     mergeSort(a, left, mid, tmp);
     mergeSort(a, mid + , right, tmp);
     int x = left;
     int y = mid + ;
     int start = left;
     while (x <= mid && y <= right)
     {
         if (a[x] < a[y])
         {
             tmp[start++] = a[x++];
         }
         else
         {
             int pos = upper_bound(a + x, a + mid + ,  * a[y]) - a;
             cnt += mid - pos + ;
             tmp[start++] = a[y++];
         }
     }
     while(x <= mid)
     {
         tmp[start++] = a[x++];
     }
     while(y <= right)
     {
         tmp[start++] = a[y++];
     }
     for (int i = left; i <= right; i++)
     {
         a[i] = tmp[i];
     }
 }
 int main()
 {
     scanf("%d", &n);
     for (int i = ; i < n; i++)
     {
         scanf("%d", &a[i]);
     }
     mergeSort(a, , n - , tmp);
     cout << cnt << endl;
     return ;
 }