樹狀數組
void sol()
{
for(int i=1;i<=n;++i) bs[i]=0;
for(int i=1;i<=n;++i)
scanf("%d",a+i),g[i]=a[i];
sort(g+1,g+1+n);
for(int i=1;i<=n;++i)
a[i]=n+1-(lower_bound(g+1,g+1+n,a[i])-g);
ll ans=0;
for(int i=1;i<=n;++i)
{
for(int j=a[i]-1;j>=1;j-=j&-j) ans+=bs[j];
for(int j=a[i];j<=n;j+=j&-j) ++bs[j];
}
printf("%lld\n",ans);
}歸併
const int N = 1010;
int a[N];
int c[N];
int cnt = 0;
void MergeSort(int l, int r)
{
int mid, i, j, tmp;
if (r > l + 1)
{
mid = (l + r) / 2;
MergeSort(l, mid);
MergeSort(mid, r);
tmp = l;
for (i = l, j = mid; i < mid && j < r;)
{
if (a[i] > a[j])
{
c[tmp++] = a[j++];
cnt += mid - i;
}
else
{
c[tmp++] = a[i++];
}
}
if (j < r)
{
for (; j < r; ++j)
{
c[tmp++] = a[j];
}
}
else
{
for (; i < mid; ++i)
{
c[tmp++]=a[i];
}
}
for (i = l; i < r; ++i)
{
a[i] = c[i];
}
}
return ;
}