gallery.cpp

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <limits.h>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <algorithm>
#include <string>
#include <functional>
#include <vector>
#include <numeric>
#include <deque>
#include <utility>
#include <bitset>
#include <iostream>
using namespace std;
typedef long long lint;
typedef long double llf;
typedef pair<int, int> pi;
 
int n, s, e;
pi a[100005];
bool rev;
 
bool ccw(pi a, pi b, pi c){
    int dx1 = b.first - a.first;
    int dy1 = b.second - a.second;
    int dx2 = c.first - a.first;
    int dy2 = c.second - a.second;
    return 1ll * dx1 * dy2 >= 1ll * dy1 * dx2;
}
 
int main(){
    scanf("%d",&n);
    for(int i=0; i<n; i++){
        scanf("%d %d",&a[i].first, &a[i].second);
    }
    a[n] = a[0];
    scanf("%d %d",&s,&e);
    if(s == 0) s = n;
    if(e == 0) e = n;
    if(s > e) swap(s, e), rev = 1;
    vector<int> ret;
    for(int i=s; i<=e; i++){
        while(ret.size() >= 2 && ccw(a[ret[ret.size()-2]], a[ret.back()], a[i])){
            ret.pop_back();
        }
        ret.push_back(i);
    }
    if(rev) reverse(ret.begin(), ret.end());
    printf("%d\n",ret.size());
    for(auto &i : ret){
        printf("%d ",i%n);
    }
}