Submission #869441

#	Time	Username	Problem	Language	Result	Execution time	Memory
869441		LeThanhMinh	Network (BOI15_net)	C++14	63 / 100	21 ms	42064 KiB

net

// Problem: H. Hai đường đi
// Contest: Codeforces - 2023 winter contest #41, Dalat winter training camp, round 1
// URL: https://gspvh23.contest.codeforces.com/group/FIWwc7nF6c/contest/484130/problem/H
// Memory Limit: 512 MB
// Time Limit: 2250 ms
// 
// Powered by CP Editor (https://cpeditor.org)

/*CP template version 3.0 - Laconic*/

#include <bits/stdc++.h>

#define ii pair<int, int>
#define fs first
#define sc second

//use struct instead of pair<int, ii>

#define int long long

#define show(v) {for (auto i : v) cout << i << " "; cout << endl;}
#define showlr(v, l, r) {for (int i = l; i <= r; i++) cout << v[i] << " "; cout << endl;}
#define all(v) v.begin(), v.end()
#define SUnique(v) {sort(all(v)); v.erase(unique(all(v)), v.end());}
#define forlr(i, l, r) for (int i = l; i <= r; i++)
#define forrl(i, r, l) for (int i = r; i >= l; i--)

#pragma GCC Optimize("O2")
#define endl "\n"
#define precise(x) cout << fixed << setprecision(x);

using namespace std;

const int N = 3e5 + 100;
const int MOD = 1e9 + 7;
const int INF = 1e9;
const int LOG = 25;
const long long LINF = 1e15 + 100;

const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, -1, 0, 1};
 
ostream& operator << (ostream &os, ii a) {
    
    os << a.fs << ' ' << a.sc;
    return os;
    
}

int binPow(int a, int b, int m = LLONG_MAX) {
    
    int prod = 1;
    a %= m;
    
    while (b) {
        
        if (b & 1) prod = prod * a % m;
        a = a * a % m;
        b /= 2;
        
    }
    
    return prod;
    
}

void setIO(string s) {
    
    if (s.empty()) return;
    
    freopen((s + ".inp").c_str(), "r", stdin);
    freopen((s + ".out").c_str(), "w", stdout);
    
}

//Try Brute Force (maybe there is a pattern)
//Try DP (Weird constraints (sum of all elements <= 10000), small constraints)
//Try Graph (a pair of number, condition between elements)
//Try Geometry (Same as graph, for coordinate)

//Try switching between problem
//Put down what you have on paper
//IMPORTANT: WRITE DOWN THE FORMULA

int n, m, q, k;
int arr[N];
vector<int> adj[N], adj1[N];

int low[N], num[N], head[N], timeDfs;
stack<int> st;
int bccId[N], bcc;

void tarjan(int u, int pre) {
    
    st.push(u);
    num[u] = low[u] = ++timeDfs;
     
    for (int v : adj[u]) {
        
        if (v == pre) continue;
        if (num[v]) {
            
            low[u] = min(low[u], num[v]);
            
        } else {
            
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
 
        }
        
    }
    
    if (num[u] == low[u]) {
        
        bcc++;
        int tmp;
        head[bcc] = u;
        
        do {
            
            tmp = st.top();
            bccId[tmp] = bcc;
            st.pop();
            num[u] = INF;
            
        } while (tmp != u);

    }
    
}

vector<int> leaf;

void dfs1(int u, int pre) {
	
	if (adj1[u].size() == 1) {
		
		leaf.push_back(u);
		if (u != pre) return;
		
	}
	
	for (int v : adj1[u]) {
		
		if (v == pre) continue;
		dfs1(v, u);
		
	}
	
}

mt19937 rng(chrono::steady_clock().now().time_since_epoch().count());

ii edges[N];

void solve() {
	
	cin >> n;
	forlr(i, 1, n) adj[i].clear(), adj1[i].clear();
	leaf.clear();
	fill(bccId + 1, bccId + n + 1, 0);
	fill(low + 1, low + n + 1, 0);
	fill(num + 1, num + n + 1, 0);
	fill(head + 1, head + n + 1, 0);
	timeDfs = 0;
	bcc = 0;
	while (st.size()) st.pop();
	
	forlr(i, 1, n - 1) {
		
		int u, v; cin >> u >> v;
		edges[i] = {u, v};
		adj[u].push_back(v);
		adj[v].push_back(u);
		
	}
	
	tarjan(1, 1);
		
	forlr(i, 1, n - 1) {
		
		auto [u, v] = edges[i];			
		int idU = bccId[u], idV = bccId[v];
		if (idU == idV) continue;
		adj1[idU].push_back(idV);
		adj1[idV].push_back(idU);
		
	}
	
	forlr(i, 1, n) SUnique(adj1[i]);
	
	dfs1(1, 1);
	
	vector<ii> res;
	
	int sz = leaf.size();
	if (sz == 2) {
		
		cout << 1 << endl;
		cout << head[leaf[0]] << " " << head[leaf[1]] << endl;
		return;
		
	}
	
	if (sz <= 1) {
		
		cout << 0 << endl;
		return;
		
	}
	
	forlr(i, 0, (sz - 1) / 2) res.push_back({head[leaf[i]], head[leaf[i + sz / 2]]});
	
	cout << res.size() << endl;
	for (ii i : res) cout << i << endl;
    
}

signed main() {
    
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);    
    // setIO("biconnected");
    
    int T = 1;
    
    bool multiTest = 0;
    if (multiTest) cin >> T;
        
    while (T--) {
        
        solve();
    
    }
    
}

Compilation message (stderr)

net.cpp:28: warning: ignoring '#pragma GCC Optimize' [-Wunknown-pragmas]
   28 | #pragma GCC Optimize("O2")
      | 
net.cpp: In function 'void solve()':
net.cpp:183:8: warning: structured bindings only available with '-std=c++17' or '-std=gnu++17'
  183 |   auto [u, v] = edges[i];
      |        ^
net.cpp: In function 'void setIO(std::string)':
net.cpp:71:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   71 |     freopen((s + ".inp").c_str(), "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net.cpp:72:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   72 |     freopen((s + ".out").c_str(), "w", stdout);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 18 / 18
• Subtask 2: 45 / 45
• Subtask 3: 0 / 37