Submission #367650

#	Time	Username	Problem	Language	Result	Execution time	Memory
367650		534351	Aesthetic (NOI20_aesthetic)	C++17	0 / 100	987 ms	70880 KiB

Aesthetic

#include <bits/stdc++.h>

using namespace std;

template<class T, class U>
void ckmin(T &a, U b)
{
    if (a > b) a = b;
}

template<class T, class U>
void ckmax(T &a, U b)
{
    if (a < b) a = b;
}

#define MP make_pair
#define PB push_back
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define SZ(x) ((int) (x).size())
#define ALL(x) (x).begin(), (x).end()
#define FOR(i, a, b) for (auto i = (a); i < (b); i++)
#define FORD(i, a, b) for (auto i = (a) - 1; i >= (b); i--)

const int MAXN = 3e5 + 13;
const long long LLINF = 3e18;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
typedef pair<pll, int> ppi;

int N, M, C, T;
vector<ppi> edge[MAXN];
ll dist[2][MAXN];
ll D;
int st[MAXN], ft[MAXN];
bitset<MAXN> seen;
vi bcc[MAXN];
vpi edges;

void dijkstra(int u)
{
    bool b = (u > 0);
    fill(dist[b], dist[b] + N + 1, LLINF);
    dist[b][u] = 0;
    priority_queue<pll, vector<pll>, greater<pll> > pq;
    pq.push({0, u});
    while(!pq.empty())
    {
        int d = pq.top().fi, u = pq.top().se; pq.pop();
        for (auto e : edge[u])
        {
            int v = e.se, nd = d + e.fi.fi;
            if (nd < dist[b][v])
            {
                dist[b][v] = nd;
                pq.push({nd, v});
            }
        }
    }
}

void dfs(int u, int p)
{
	st[u] = T;
	ft[u] = T;
	T++;
	seen[u] = true;
	for (auto e : edge[u])
	{
        int v = e.se, d = e.fi.fi;
        //if this is not on any shortest paths we just ignore it.
        if (dist[0][u] + d + dist[1][v] >= D && dist[1][u] + d + dist[0][v] >= D) continue;
        //oh no it is part of a path shorter than D so we legit care about it.
		if (v == p) continue;
		if (seen[v])
		{
			if (st[v] > st[u]) continue;
			edges.PB({u, v});
			ckmin(ft[u], st[v]);
			continue;
		}
		edges.PB({u, v});
		dfs(v, u);
		ckmin(ft[u], ft[v]);
		if (ft[v] >= st[u])
		{
			while(!edges.empty())
			{
				pii p = edges.back();
				edges.pop_back();
				bcc[C].PB(p.fi);
				bcc[C].PB(p.se);
				if (p == MP(u, v)) break;
			}
			C++;
		}
	}
}

bool check()
{
    //can you make the dist >D?
    //ans is YES if there's a bridge & this thing extends dist by more than D.
    T = 0;
    fill(st, st + N, 0);
    fill(ft, ft + N, 0);
    seen.reset();
    FOR(i, 0, C) bcc[i].clear();
    C = 0;
    FOR(i, 0, N)
    {
        if (seen[i]) continue;
        dfs(i, N);
    }
    set<pii> bridges;
    FOR(i, 0, C)
    {
        sort(ALL(bcc[i]));
        bcc[i].erase(unique(ALL(bcc[i])), bcc[i].end());
        if (SZ(bcc[i]) == 2)
        {
            bridges.insert({bcc[i][0], bcc[i][1]});
        }
    }
    FOR(u, 0, N)
    {
        for (auto e : edge[u])
        {
            int v = e.se, d = e.fi.fi, mx = e.fi.se;
            if (bridges.find({u, v}) == bridges.end()) continue;
            if (dist[0][u] + mx + dist[1][v] >= D && dist[1][u] + mx + dist[0][v] >= D) return true;
        }
    }
    return false;
    //now just check if there's something we can extend that is a bridge.
}

int32_t main()
{
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(4);
    ios_base::sync_with_stdio(false); cin.tie(0);
    cin >> N >> M;
    vector<array<int, 3> > eds(M);
    FOR(i, 0, M)
    {
        int a, b, w;
        cin >> a >> b >> w; a--; b--;
        eds[i] = {a, b, w};
    }
    reverse(ALL(eds));
    int mx = 0;
    for (auto e : eds)
    {
        int a = e[0], b = e[1], w = e[2];
        edge[a].PB({{w, w + mx}, b});
        edge[b].PB({{w, w + mx}, a});
        ckmax(mx, w);
    }
    dijkstra(0);
    dijkstra(N - 1);
    ll lo = dist[0][N - 1], hi = 2 * lo + 1;
    while(hi > lo)
    {
        D = (hi + lo + 1) >> 1;
        if (check()) lo = D;
        else hi = D - 1;
    }
    cout << lo << '\n';
    return 0;
}

Compilation message (stderr)

Aesthetic.cpp: In function 'bool check()':
Aesthetic.cpp:139:27: warning: unused variable 'd' [-Wunused-variable]
  139 |             int v = e.se, d = e.fi.fi, mx = e.fi.se;
      |                           ^

• Subtask 1: 0 / 5
• Subtask 2: 0 / 8
• Subtask 3: 0 / 7
• Subtask 4: 0 / 15
• Subtask 5: 0 / 16
• Subtask 6: 0 / 22
• Subtask 7: 0 / 27