歸併排序求逆序對

 1 #include <iostream>
 2 #include <algorithm>
 3 #include <cstring>
 4 #include <cstdio>
 5 
 6 typedef long long LL;
 7 const int MAX_N = (int)5e5 + 10;
 8 
 9 int n;
10 LL a[MAX_N], tmpA[MAX_N], cnt = 0;
11 void merge_sort(int l, int r, LL *A) {
12     if
 
 (l >= r) return;
13 
14     int mid = (l + r) >> 1;
15     merge_sort(l, mid , A);
16     merge_sort(mid + 1, r, A);
17 
18     int sl = l, sr = mid + 1, tmp = 0;
19     while (sl <= mid && sr <= r) {
20         if (A[sl] <= A[sr]) tmpA[tmp++] = A[sl++];
21         else
 
 tmpA[tmp++] = A[sr++], cnt += mid - sl + 1;
22     }
23     while (sl <= mid) tmpA[tmp++] = A[sl++];
24     while (sr <= r) tmpA[tmp++] = A[sr++];
25     for (int i = 0; i < tmp; i++) A[i + l] = tmpA[i];
26 }
27 
28 int main() {
29     scanf("%d", &n);
30     for (int i = 1; i <= n; i++) scanf("
 
%lld", &a[i]);
31 
32     merge_sort(1, n, a);
33     printf("%lld\n", cnt);
34 
35     return 0;
36 }