hihoCoder#1141 二分·归并排序之逆序对

 1 #include <iostream>
 2 #include <cstring>
 3 
 4 using namespace std;
 5 
 6 #define MAX_N 100008
 7 
 8 int N;
 9 long long a[MAX_N];
10 long long b[MAX_N];
11 
12 long long count(int l, int r) {
13   if (l >= r)
14     return 0;
15 
16   long long res = 0;
17   int m = (l + r) / 2;
18   res += count(l, m);
19   res += count(m + 1, r);
20   int i = l;
21   int j = m + 1;
22   int k = 0;
23   while (i <= m && j <= r) {
24     if (a[i] <= a[j]) {
25       b[k++] = a[i++];
26     }
27     else {
28       res += (m - i + 1);
29       b[k++] = a[j++];
30     }
31   }
32   while (i <= m)
33     b[k++] = a[i++];
34   while (j <= r)
35     b[k++] = a[j++];
36 
37   while (k--)
38     a[l + k] = b[k];
39   return res;
40 }
41 
42 int main() {
43   scanf("%d", &N);
44   for (int i = 0; i < N; i++)
45     scanf("%lld", &(a[i]));
46   printf("%lld\n", count(0, N - 1));
47   return 0;
48 }