AC日记——逆序对 洛谷 P1908

逆序对

思路：

　　线段树水过；

代码：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

#define maxn 40005
#define ll long long

inline void in(int &now)
{
    char Cget=getchar();now=;
    while(Cget>''||Cget<'') Cget=getchar();
    while(Cget>=''&&Cget<='')
    {
        now=now*+Cget-'';
        Cget=getchar();
    }
}

inline void in(ll &now)
{
    char Cget=getchar();now=;
    while(Cget>''&&Cget<'') Cget=getchar();
    while(Cget>=''&&Cget<='')
    {
        now=now*+Cget-'';
        Cget=getchar();
    }
}

struct TreeNodeType {
    int l,r,mid,dis;

    TreeNodeType *lc,*rc;

    TreeNodeType() {lc=NULL,rc=NULL,dis=;}
};

class NiXuDuiWorkPType {
    private:
        int ai[maxn],bi[maxn],n,size;

        ll ans;

        TreeNodeType *root;

        void build(TreeNodeType *&now,int l,int r)
        {
            now=new TreeNodeType,now->l=l,now->r=r;
            if(l==r) return ;now->mid=l+r>>;
            build(now->lc,l,now->mid),build(now->rc,now->mid+,r);
        }

        void updata(TreeNodeType *now,int to)
        {
            if(now->l==now->r) {now->dis++;return;}
            if(to<=now->mid) updata(now->lc,to);else updata(now->rc,to);
            now->dis=now->lc->dis+now->rc->dis;
        }

        void query(TreeNodeType *now,int l,int r)
        {
            if(now->l>=l&&now->r<=r) {ans+=now->dis;return;}
            if(l<=now->mid) query(now->lc,l,min(now->mid,r));
            if(r>now->mid) query(now->rc,max(now->mid+,l),r);
        }

    public:
        NiXuDuiWorkPType()
        {
            in(n);for(int i=;i<=n;i++) in(ai[i]),bi[i]=ai[i];
            sort(bi+,bi+n+),size=unique(bi+,bi+n+)-bi-;build(root,,size);
            for(int i=;i<=n;i++)
            {
                ai[i]=lower_bound(bi+,bi+size+,ai[i])-bi;
                if(ai[i]<size) query(root,ai[i]+,size);updata(root,ai[i]);
            }
            cout<<ans;
        }
};
class NiXuDuiWorkPType pos;

int main()
{
    return ;
}