树状数组求逆序对 基础版+强化版本（去重+离散化）

 

是getsum(a[i]-1)

不是getsum(i-1);

20210920

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e5;
int a[maxn],n,c[maxn],ans[maxn];
int lowbit(int x)
{
    return x&(-x);
}
int update(int id,int val)
{
    a[id]=val;
    while(id<=n)
     {  
         c[id]+=val;
         id+=lowbit(id);
     }
}
int getsum(int x)
{ int ans=0;
    while(x>0)
     {
         ans+=c[x];
         x-=lowbit(x);
     }
     return ans;
}
int main( )
{

   cin>>n;
   for(int i=1;i<=n;i++)
    {
        cin>>a[i];
    }
    

    
    for(int i=1;i<=n;i++)
    {
        update(a[i],1);
        ans[i]=i-getsum(a[i]-1);
        cout<<ans[i]<<" ";
    }    
    
}

 

强化版本，自己想的+手写的，因为看不懂也不想看洛谷那群家伙写的代码

 

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#include<bits/stdc++.h> using namespace std; const int maxn=1e6; long long b[maxn],n,c[maxn],vis[maxn]; struct node{     long long val,id; }a[maxn]; bool cmp(node a,node b) {     return a.val<b.val; } int lowbit(int x) {     return x&(-x); } void update(int id,int val) {     while(id<=n)      {          c[id]+=val;         id+=lowbit(id);      } } long long getsum(int x) { long long ans=0;     while(x>0)      {         ans+=c[x];         x-=lowbit(x);      }      return ans; } void pre( ) {   memset(vis,0,sizeof(vis));   memset(c,0,sizeof(c)); } int main( ) {                //freopen("lyslys.in","r",stdin); int t; cin>>t; while(t--) {     pre( );    cin>>n;     for(int i=1;i<=n;i++)     {   cin>>a[i].val;         a[i].id=i;         //cout<<a[i]<<endl;     /*  update(a[i],1);         int x=i-getsum(a[i]-1)-1;        /* int y;         y=i-1-x;         if(x>y)         {             ans[i]=1;             //逆序的更多，放前面贪心         }         else {             ans[i]=0;         }         //这边要不要讨论==0的情况         */     }               sort(a+1,a+1+n,cmp);                  int i,j;      long long cnt=0;            for(i=1;i<=n;)     {   cnt++;         for(j=i+1;j<=n;j++)          {             if(a[j].val==a[i].val)              {                a[j].val=cnt;              }              else break;          }          a[i].val=cnt;          i=j;     }           for(int k=1;k<=n;k++)     {         b[a[k].id]=a[k].val;     }                 cnt=0;                 for(int i=1;i<=n;i++)     {           //cout<<a[i]<<endl;         update(b[i],1);         vis[b[i]]++;                   long long x,y;                   x=i-getsum(b[i]-1)-vis[b[i]];                    y=getsum(b[i]-1);                  // cout<<x<<" "<<y<<endl;                   cnt+=min(x,y);        /* int y;         y=i-1-x;         if(x>y)         {             ans[i]=1;             //逆序的更多，放前面贪心         }         else {             ans[i]=0;         }         //这边要不要讨论==0的情况         */        //cout<<x<<" ";     }     printf("%lld\n",cnt); }   }