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63 lines
1.5 KiB
C++
63 lines
1.5 KiB
C++
#include <cstdio>
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inline int min(int a, int b) { return a < b ? a : b; }
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int head[10010], next[50010], to[50010], ecnt;
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int dfn[10010], low[10010], stk[10010], scc[10010], top, idx, scccnt;
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bool instk[10010];
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int deg[10010];
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inline void addEdge(int f, int t)
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{
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ecnt++;
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next[ecnt] = head[f];
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head[f] = ecnt;
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to[ecnt] = t;
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}
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void tarjan(int x)
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{
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dfn[x] = low[x] = ++idx;
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instk[stk[top++] = x] = true;
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for (int cur = head[x]; cur; cur = next[cur])
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if (!dfn[to[cur]]) tarjan(to[cur]), low[x] = min(low[x], low[to[cur]]);
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else if (instk[to[cur]])
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low[x] = min(low[x], dfn[to[cur]]);
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if (dfn[x] == low[x])
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{
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scccnt++;
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do
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{
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top--;
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scc[stk[top]] = scccnt;
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instk[stk[top]] = false;
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} while (stk[top] != x);
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}
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}
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int main()
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{
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int n, m;
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scanf("%d%d", &n, &m);
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for (int i = 0, x, y; i < m; i++)
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{
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scanf("%d%d", &x, &y);
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addEdge(x, y);
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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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for (int i = 1; i <= n; i++)
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for (int cur = head[i]; cur; cur = next[cur])
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if (scc[i] != scc[to[cur]])
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deg[scc[i]]++;
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int zcnt = 0, id = 0;
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for (int i = 1; i <= scccnt; i++)
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if (deg[i] == 0)
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zcnt++, id = i;
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if (zcnt != 1) putchar('0');
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else
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{
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int ans = 0;
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for (int i = 1; i <= n; i++)
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if (scc[i] == id)
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ans++;
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printf("%d", ans);
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}
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return 0;
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} |