poj 2104（主席树）

#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
int p,node_cnt,n,m;
int a[200005],b[200005],rt[6000005],lc[6000005],rc[6000005],sum[6000005];
void build(int&t,int l,int r){
    t = ++node_cnt;
    if(l==r)return;
    int mid = (l+r)>>1;
    build(lc[t],l,mid);
    build(rc[t],mid+1,r);
}
int modify(int o,int l,int r){
    int oo = ++node_cnt;
    sum[oo] = sum[o]+1;
    if(l==r)return oo;
    lc[oo] = lc[o];
    rc[oo] = rc[o]; 
    int mid = (l+r)>>1;
    if(p<=mid){
        lc[oo] = modify(lc[oo],l,mid);
    }
    else{
        rc[oo] = modify(rc[oo],mid+1,r);
    }
    return oo;
}
int query(int u,int v,int l,int r,int k){
    int ans,mid=((l+r)>>1),x = sum[lc[v]]-sum[lc[u]];
    if(l==r)return l;
    if(x>=k){
        ans = query(lc[u],lc[v],l,mid,k);
    }
    else{
        ans = query(rc[u],rc[v],mid+1,r,k-x);
    }
    return ans;
}
int main(){
    int l,r,k,ans,q;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
        b[i] = a[i];
    }
    sort(b+1,b+n+1);
    q = unique(b+1,b+n+1)-b-1;
    build(rt[0],1,q);
    for(int i=1;i<=n;i++){
        p = lower_bound(b+1,b+q+1,a[i])-b;
        rt[i] = modify(rt[i-1],1,q);
    }
    while(m--){
        scanf("%d%d%d",&l,&r,&k);
        ans = query(rt[l-1],rt[r],1,q,k);
        printf("%d\n",b[ans]);
    }
    return 0;
}