mirror of
https://git.sb/baoshuo/OI-codes.git
synced 2024-11-24 00:28:48 +00:00
79 lines
1.4 KiB
C++
79 lines
1.4 KiB
C++
#include <iostream>
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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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using std::endl;
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// Limits
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const int N = 10005;
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int n, m, a, b;
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std::vector<int> g[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];
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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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int main() {
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cin >> n >> m;
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for (int i = 1; i <= m; i++) {
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cin >> a >> b;
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g[a].push_back(b);
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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[i] != id[v]) {
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dout[id[i]]++;
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}
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}
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}
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int z = 0, sum = 0;
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for (int i = 1; i <= scc_cnt; i++) {
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if (!dout[i]) {
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z++;
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sum += siz[i];
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if (z > 1) {
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sum = 0;
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break;
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}
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}
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}
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cout << sum << endl;
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return 0;
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}
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