USACO 2015 Dec Platinum 2

#include <algorithm>
#include <iostream>
#include <vector>
#include <set>

using namespace std;

int main() {
    FILE *fin = fopen("cardgame.in", "r");
    FILE *fout = fopen("cardgame.out", "w");
    int n;
    fscanf(fin, "%d", &n);

    vector<int> elsie(n);
    for (int i = 0; i < n; i++) {
        int e;
        fscanf(fin, "%d", &e);
        elsie[i] = e;
    }

    set<int> elsieset(elsie.begin(), elsie.end());

    set<int> bessie;
    int preve = 0;
    for (int e : elsieset) {
        for (int i = preve + 1; i < e; i++) {
            bessie.insert(i);
        }
        preve = e;
    }
    for (int i = preve + 1; i < 2 * n + 1; i++) {
        bessie.insert(i);
    }

    set<int> bessie_tmp(bessie);
    vector<int> rupper(n);
    int wupper = 0;
    for (int elsiei = 0; elsiei < n; elsiei++) {
        int e = elsie[elsiei];
        set<int>::iterator touse = bessie_tmp.lower_bound(e);
        if (touse != bessie_tmp.end()) {
            bessie_tmp.erase(touse);
            wupper++;
        }
        rupper[elsiei] = wupper;
    }

    bessie_tmp = set<int>(bessie);
    vector<int> rlower(n);
    int wlower = 0;
    for (int elsiei = n - 1; elsiei >= 0; elsiei--) {
        int e = elsie[elsiei];
        set<int>::iterator touse = bessie_tmp.lower_bound(e);
        if (touse != bessie_tmp.begin()) {
            bessie_tmp.erase(--touse);
            wlower++;
        }
        rlower[elsiei] = wlower;
    }

    int best = 0;
    for (int i = -1; i < n; i++) {
        best = max(best, (i >= 0 ? rupper[i] : 0) + (i + 1 < n ? rlower[i + 1] : 0));
    }

    fprintf(fout, "%d\n", best);
    return 0;
}