復習主席樹，這東西掌握得不好。

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
struct chair
{
    int lc,rc,c;
}tr[21000000];int cnt,rt[510000];
int maketree(int x,int l,int r,int p)
{
    if(x==0)x=++cnt;
    tr[x].c++;
    if(l<r)
    {
        int mid=(l+r)/2
 
;
        if(p<=mid)tr[x].lc=maketree(tr[x].lc,l,mid,p);
        else      tr[x].rc=maketree(tr[x].rc,mid+1,r,p);
    }
    return x;
}
int merge(int x,int y)
{
    if(x==0||y==0)return x+y;
    tr[x].c+=tr[y].c;
    tr[x].lc=merge(tr[x].lc,tr[y].lc);
    tr[x].rc=merge(tr[x].rc,tr[y].rc);
    
 
return x;
}
bool bk;
void ask(int x,int y,int l,int r,int k)
{
    if(bk==true)return ;
    if(tr[x].c-tr[y].c<k)return ;
    if(l==r){bk=true;printf("%d\n",l);return ;}
    int mid=(l+r)/2;
    ask(tr[x].lc,tr[y].lc,l,mid,k);
    ask(tr[x].rc,tr[y].rc,mid+1,r,k);
     
}
int a[510000];
 
int main()
{
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++)scanf("%d",&a[i]);
     
    cnt=0;memset(rt,0,sizeof(rt));
    for(int i=1;i<=n;i++)
    {
        rt[i]=maketree(rt[i],1,500000,a[i]);
        rt[i]=merge(rt[i],rt[i-1]);
    }
     
    int x,y;
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&x,&y);
        bk=false;
        ask(rt[y],rt[x-1],1,500000,(y-x+1)/2+1);
        if(bk==false)printf("0\n");
    }
    return 0;
}

bzoj3524: [Poi2014]Couriers