lessons/参考实现/02-强连通与双连通/POJ-3177.cpp

#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;
struct E { int to, id; };

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, m; cin >> n >> m;
    vector<vector<E>> g(n);
    vector<pair<int, int>> edges(m);
    for (int id = 0; id < m; ++id) {
        int u, v; cin >> u >> v; --u; --v; edges[id] = {u, v};
        g[u].push_back({v, id}); g[v].push_back({u, id});
    }
    vector<int> dfn(n), low(n), par(n, -1), pe(n, -1);
    vector<size_t> it(n); vector<bool> bridge(m); int timer = 0;
    for (int root = 0; root < n; ++root) if (!dfn[root]) {
        vector<int> st{root}; dfn[root] = low[root] = ++timer;
        while (!st.empty()) {
            int u = st.back();
            if (it[u] < g[u].size()) {
                E e = g[u][it[u]++];
                if (e.id == pe[u]) continue;
                if (!dfn[e.to]) {
                    par[e.to] = u; pe[e.to] = e.id;
                    dfn[e.to] = low[e.to] = ++timer; st.push_back(e.to);
                } else low[u] = min(low[u], dfn[e.to]);
            } else {
                st.pop_back();
                if (par[u] != -1) {
                    if (low[u] > dfn[par[u]]) bridge[pe[u]] = true;
                    low[par[u]] = min(low[par[u]], low[u]);
                }
            }
        }
    }
    vector<int> comp(n, -1); int count = 0;
    for (int s = 0; s < n; ++s) if (comp[s] == -1) {
        vector<int> st{s}; comp[s] = count;
        while (!st.empty()) {
            int u = st.back(); st.pop_back();
            for (E e : g[u]) if (!bridge[e.id] && comp[e.to] == -1) {
                comp[e.to] = count; st.push_back(e.to);
            }
        }
        ++count;
    }
    vector<int> degree(count);
    for (int id = 0; id < m; ++id) if (bridge[id]) {
        auto [u, v] = edges[id]; ++degree[comp[u]]; ++degree[comp[v]];
    }
    int leaves = 0;
    for (int d : degree) leaves += d == 1;
    cout << (leaves + 1) / 2 << '\n';
}