hihoCoder#1141 二分·归并排序之逆序对

原题地址

又是一道WA成狗的题，最后发现原来是结果溢出了。。

代码：

 #include <iostream>
 #include <cstring>

 using namespace std;

 #define MAX_N 100008

 int N;
 long long a[MAX_N];
 long long b[MAX_N];

 long long count(int l, int r) {
   if (l >= r)
     return ;

   long long res = ;
   int m = (l + r) / ;
   res += count(l, m);
   res += count(m + , r);
   int i = l;
   int j = m + ;
   int k = ;
   while (i <= m && j <= r) {
     if (a[i] <= a[j]) {
       b[k++] = a[i++];
     }
     else {
       res += (m - i + );
       b[k++] = a[j++];
     }
   }
   while (i <= m)
     b[k++] = a[i++];
   while (j <= r)
     b[k++] = a[j++];

   while (k--)
     a[l + k] = b[k];
   return res;
 }

 int main() {
   scanf("%d", &N);
   for (int i = ; i < N; i++)
     scanf("%lld", &(a[i]));
   printf("%lld\n", count(, N - ));
   return ;
 }