mirror of
https://git.sb/baoshuo/OI-codes.git
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93 lines
1.7 KiB
C++
93 lines
1.7 KiB
C++
#include <iostream>
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#include <cstring>
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#include <stack>
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#include <vector>
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using std::cin;
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using std::cout;
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const char endl = '\n';
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const int N = 2e5 + 5;
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int n, m, t[N], ans;
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std::vector<int> g[N], g2[N];
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// Tarjan
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int cnt, dfn[N], low[N];
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int scc_cnt, id[N], siz[N];
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int dout[N], din[N];
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bool vis[N];
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std::stack<int> st;
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void tarjan(int u) {
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dfn[u] = low[u] = ++cnt;
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st.push(u);
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vis[u] = true;
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for (int v : g[u]) {
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if (!dfn[v]) {
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tarjan(v);
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low[u] = std::min(low[u], low[v]);
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} else if (vis[v]) {
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low[u] = std::min(low[u], dfn[v]);
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}
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}
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if (dfn[u] == low[u]) {
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scc_cnt++;
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int v;
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do {
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v = st.top();
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st.pop();
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vis[v] = false;
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id[v] = scc_cnt;
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siz[scc_cnt]++;
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} while (v != u);
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}
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}
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void dfs(int u) {
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for (int v : g2[u]) {
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if (t[v] == -1) dfs(v);
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if (t[v] == 1 || siz[u] > 1) t[u] = 1;
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}
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if (siz[u] > 1) t[u] = 1;
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if (t[u] == -1) t[u] = 0;
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}
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int main() {
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std::ios::sync_with_stdio(false);
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memset(t, 0xff, sizeof(t));
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cin >> n >> m;
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for (int i = 1, u, v; i <= m; i++) {
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cin >> u >> v;
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g[u].push_back(v);
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}
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for (int i = 1; i <= n; i++) {
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if (!dfn[i]) {
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tarjan(i);
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}
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}
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for (int i = 1; i <= n; i++) {
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for (int v : g[i]) {
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if (id[v] != id[i]) {
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din[id[v]]++;
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g2[id[i]].push_back(id[v]);
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}
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}
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}
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for (int i = 1; i <= scc_cnt; i++) {
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if (!din[i]) dfs(i);
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}
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for (int i = 1; i <= scc_cnt; i++) {
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if (t[i] == 1) ans += siz[i];
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}
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cout << ans << endl;
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return 0;
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}
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