20201009 day30 复习4:逆序对
1 归并排序
#include<cstdio>
#define msort merge_sort
#define ll long long
using namespace std;
const int maxn=5e5+5;
int a[maxn],r[maxn],n;
long long ans=0;
void msort(int s,int t){
if(s==t) return ;
int mid=(s+t)/2;
msort(s,mid);
msort(mid+1,t);
int i=s,j=mid+1,k=s;
while(i<=mid&&j<=t)
if(a[i]<=a[j]) r[k]=a[i],k++,i++;//先赋值再+1
else r[k]=a[j],k++,j++,ans+=(ll)mid-i+1;//理解为数学归纳
while(i<=mid) r[k]=a[i],k++,i++;
while(j<=t) r[k]=a[j],k++,j++;
for(int i=s;i<=t;i++) a[i]=r[i];
}
int main(){
scanf("%d",&n);
for(int i=1;i<=n;i++){
scanf("%d",&a[i]);
}
msort(1,n);
printf("%lld\n",ans);
return 0;
}
2 离散化树状数组
#include<iostream>
#include<cstdio>
#include<algorithm>
#define ll long long
using namespace std;
const int maxx=1000005;
ll a[maxx],b[maxx],s[maxx];
ll n;
void init()
{
sort(b+1,b+1+n);
unique(b+1,b+1+n);//-b-1;
for(ll i=1;i<=n;i++)
{
a[i]=lower_bound(b+1,b+n+1,a[i])-b;
}
return;
}
int main()
{
scanf("%lld",&n);
for(ll i=1;i<=n;i++)
{
scanf("%lld",&a[i]);
b[i]=a[i];
}
ll ans=0;
init();
for(ll i=n;i>=1;i--)
{
for(ll j=a[i];j<=n;j+=(j&-j))
{
s[j]++;
}
for(ll j=a[i]-1;j>=1;j-=(j&-j))
{
ans+=s[j];
}
}
printf("%lld",ans);
return 0;
}
要做就做南波万