SAPO 2018 stadium

#include <bits/stdc++.h>
#define pi pair<int,int>
#define x first
#define y second

using namespace std;

class fcmp {
    public:
        bool operator()(const pi &a, const pi &b) const {
            return tie(a.y, a.x) < tie(b.y, b.x);
        };
};

void max_self(int &a, int b) {
    a = max(a, b);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int N, W;
    cin >> N >> W;
    vector<pi> trees(N);
    for(pi &x : trees)
        cin >> x.x >> x.y;
    sort(trees.begin(), trees.end());
    set<pi,fcmp> edges{{-1, 0}, {W + 1, W}};
    multiset<int> gaps{W};
    int ans = 0, p = 0;
    for(int i = -1; i < N; ++i) {
        int x0 = 0;
        if(i != -1) {
            const pi &t = trees[i];
            x0 = t.x;
            auto pos = edges.find(t);
            auto a = prev(pos);
            auto b = next(pos);
            cout << i << endl;
            cout << "a: " << a->x << ' ' << a->y << endl;
            cout << "b: " << b->x << ' ' << b->y << endl;
            gaps.erase(gaps.find(t.y - a->y));
            gaps.erase(gaps.find(b->y - t.y));
            gaps.insert(b->y - a->y);
            edges.erase(pos);
        }
        if(p < i)
            p = i;
        int x1 = (p == N) ? W : trees[p].x;
        max_self(ans, min(x1 - x0, *gaps.rbegin()));
        while(p < N && trees[p].x - x0 <= *gaps.rbegin()) {
            cout << *gaps.rbegin() << endl;
            const pi &t = trees[p];
            auto b = edges.lower_bound(t);
            auto a = prev(b);
            gaps.erase(gaps.find(b->y - a->y));
            gaps.insert(t.y - a->y);
            gaps.insert(b->y - t.y);
            edges.insert(t);
            cout << i << ' ' << p << endl;
            cout << "t: " << t.x << ' ' << t.y << endl;
            cout << "b: " << b->x << ' ' << b->y << endl;
            cout << "a: " << a->x << ' ' << a->y << endl;
            cout << "dif " << b->y - a->y << endl;
            cout << "t.y-a->y " << t.y - a->y << endl;
            cout << "b->y-t.y " << b->y - t.y << endl;
            cout << *gaps.rbegin() << endl;
            cout << endl << endl;
            ++p;
            x1 = (p == N) ? W : trees[p].x;
            max_self(ans, min(x1 - x0, *gaps.rbegin()));
        }
    }
    cout << ans;
}