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  1. #include <bits/stdc++.h>
  2.  
  3. using namespace std;
  4.  
  5. const int N = 1<<20;
  6.  
  7. int tests,n,a[N],b[N];
  8.  
  9. long long count_inversions(int l, int r) {
  10. 	if(l==r) return 0;
  11. 	int m=(l+r)>>1,i,j,sz=0;
  12. 	long long ans=0;
  13. 	ans+=count_inversions(l,m);
  14. 	ans+=count_inversions(m+1,r);
  15. 	i=l;j=m+1;
  16. 	while(i<=m && j<=r) if(a[j]<a[i]) ans+=m-i+1,b[++sz]=a[j++]; else b[++sz]=a[i++];
  17. 	while(i<=m) b[++sz]=a[i++];
  18. 	while(j<=r) b[++sz]=a[j++];
  19. 	for(i=1;i<=sz;i++) a[l+i-1]=b[i];
  20. 	return ans;
  21. }
  22.  
  23. int main() {
  24. 	int i;
  25. 	scanf("%d", &tests);
  26. 	while(tests--) {
  27. 		scanf("%d", &n);
  28. 		for(i=1;i<=n;i++) scanf("%d", &a[i]);
  29. 		printf("%lld\n", count_inversions(1,n));
  30. 	}
  31. 	return 0;
  32. }
Success #stdin #stdout 0s 11648KB
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https://ideone.com/yu7kcm
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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