[BZOJ 3524][Poi2014]Couriers：可持久化线段树

/*
User:Small
Language:C++
Problem No.:3524
*/
#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int M=5e5+5;
int n,q,cnt,rt[M];
struct no{
int ls,rs,sum;
}a[M*20];
void insert(int &rt,int pre,int l,int r,int pos){
rt=++cnt;
a[rt]=a[pre];
a[rt].sum++;
if(l==r) return;
int mid=l+r>>1;
if(pos<=mid) insert(a[rt].ls,a[pre].ls,l,mid,pos);
else insert(a[rt].rs,a[pre].rs,mid+1,r,pos);
}
int query(int s,int t){
int l=1,r=n,cnt=(t-s+1)/2;
int x=rt[s-1],y=rt[t];
while(l!=r){
int mid=l+r>>1;
if(a[y].sum-a[x].sum<=cnt) return 0;
if(a[a[y].ls].sum-a[a[x].ls].sum>cnt){
y=a[y].ls;
x=a[x].ls;
r=mid;
}
else if(a[a[y].rs].sum-a[a[x].rs].sum>cnt){
y=a[y].rs;
x=a[x].rs;
l=mid+1;
}
else return 0;
}
return l;
}
int main(){
freopen("data.in","r",stdin);//
scanf("%d%d",&n,&q);
for(int i=1;i<=n;i++){
int x;
scanf("%d",&x);
insert(rt[i],rt[i-1],1,n,x);
}
while(q--){
int l,r;
scanf("%d%d",&l,&r);
printf("%d\n",query(l,r));
}
return 0;
}