模板—主席树（待修改）

这个有点复杂，按理应该写写的……下次再说吧

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
struct node
{
    int l,r,sum;
    #define l(x) tr[x].l
    #define r(x) tr[x].r
    #define sum(x) tr[x].sum
}tr[10000000];
struct que
{
    char a;int l,r,kth;
}qq[20010];

int cnt,m,ce,T[20010],S[20010];
int n,Q,a[20010],b[20010];
int use[3][20010];

inline int lowbit(int x){return x&(-x);}
inline int build(int l,int r)
{
    int now=++cnt;
    if(l==r)return now;
    int mid=(l+r)>>1;
    l(now)=build(l,mid);
    r(now)=build(mid+1,r);
    return now;
}
inline int build_new(int flag,int mark,int loc,int val)
{
    int rt=++cnt;
    int before;
    if(mark==1)before=T[0];
    else before=flag?T[mark-1]:S[mark-1];
    if(!flag && val) before=S[mark];
    sum(rt)=sum(before)+val;
    
    int l=1,r=m,now=rt,still=before,mid;
    for(;l(still)||r(still);)
    {
        mid=(l+r)>>1;
        if(loc>mid)
        {
            l(now)=l(still);r(now)=++cnt;
            sum(cnt)=sum(r(still))+val;
            now=cnt;still=r(still);
            l=mid+1;
        }
        else
        {
            r(now)=r(still);l(now)=++cnt;
            sum(cnt)=sum(l(still))+val;
            now=cnt;still=l(still);
            r=mid;
        }
    }
    return rt;
}
inline void updata(int loc,int num)
{
    int where;
    where=lower_bound(b+1,b+m+1,a[loc])-b;
    for(int i=loc;i<=n;i+=lowbit(i))
        S[i]=build_new(0,i,where,-1);
    where=lower_bound(b+1,b+m+1,num)-b;
    for(int i=loc;i<=n;i+=lowbit(i))
        S[i]=build_new(0,i,where,1);
    a[loc]=num;
}
int Sum(int y,int x)
{
    int res=0;
    for(int i=x;i>=1;i-=lowbit(i))
        res+=sum(l(use[y][i]));
    return res;
}
int ask(int u,int v,int a,int b,int l,int r,int k)
{
    if(l==r)return l;
    int lm=Sum(2,v)+sum(l(b))-sum(l(a))-Sum(1,u),
        mid=(l+r)>>1;
    if(k<=lm)
    {
        for(int i=u;i>=1;i-=lowbit(i))
            use[1][i]=l(use[1][i]);
        for(int i=v;i>=1;i-=lowbit(i))
            use[2][i]=l(use[2][i]);
        return ask(u,v,l(a),l(b),l,mid,k);
    }
    else
    {
        for(int i=u;i>=1;i-=lowbit(i))
            use[1][i]=r(use[1][i]);
        for(int i=v;i>=1;i-=lowbit(i))
            use[2][i]=r(use[2][i]);
        return ask(u,v,r(a),r(b),mid+1,r,k-lm);
    }
}
signed main()
{
    cin>>n>>Q;ce=n;
    for(int i=1;i<=n;i++)cin>>a[i],b[i]=a[i];
    for(int i=1;i<=Q;i++)
    {
        cin>>qq[i].a>>qq[i].l>>qq[i].r;
        if(qq[i].a=='Q')cin>>qq[i].kth;
        else b[++ce]=qq[i].r;
    }
    sort(b+1,b+ce+1);
    m=unique(b+1,b+ce+1)-b-1;
    T[0]=build(1,m);
    for(int i=1;i<=n;i++)
    {
        int loc=lower_bound(b+1,b+m+1,a[i])-b;
        T[i]=build_new(1,i,loc,1);
    }
    for(int i=1;i<=n;i++)
        S[i]=build_new(0,i,1,0);
    for(int i=1;i<=Q;i++)
    {
        if(qq[i].a=='Q')
        {
            int x=qq[i].l,y=qq[i].r;
            for(int j=x-1;j>=1;j-=lowbit(j))
                use[1][j]=S[j];
            for(int j=y;j>=1;j-=lowbit(j))
                use[2][j]=S[j];            
            printf("%ld\n",b[ask(x-1,y,T[x-1],T[y],1,m,qq[i].kth)]);
        }
        else updata(qq[i].l,qq[i].r);
    }
}