LYDSY 2438
题目大意
暂无
题目解法
暂无
RTFC
#include <cstdio>
inline int min(int a, int b) { return a < b ? a : b; }
int n, m, head[100010], next[300010], to[300010], ecnt,
scc[100010], scccnt, st[100010], top, dfn[100010], low[100010], idx,
nHead[100010], nNext[300010], nTo[300010], nEcnt, in[100010], sccsz[100010];
bool instack[100010];
inline void addEdge(int f, int t)
{
ecnt++;
next[ecnt] = head[f];
head[f] = ecnt;
to[ecnt] = t;
}
inline void addEdgeN(int f, int t)
{
nEcnt++;
nNext[nEcnt] = nHead[f];
nHead[f] = nEcnt;
nTo[nEcnt] = t;
}
void tarjan(int u)
{
dfn[u] = low[u] = ++idx;
instack[st[top++] = u] = true;
for (int cur = head[u]; cur; cur = next[cur])
if (!dfn[to[cur]])
tarjan(to[cur]), low[u] = min(low[u], low[to[cur]]);
else if (instack[to[cur]])
low[u] = min(low[u], dfn[to[cur]]);
if (dfn[u] == low[u])
{
scccnt++;
do
{
top--;
scc[st[top]] = scccnt;
instack[st[top]] = false;
sccsz[scccnt]++;
} while (st[top] != u);
}
}
int main()
{
scanf("%d%d", &n, &m);
for (int i = 1, x, y; i <= m; i++)
{
scanf("%d%d", &x, &y);
addEdge(x, y);
}
for (int i = 1; i <= n; i++)
if (!dfn[i]) tarjan(i);
for (int i = 1; i <= n; i++)
for (int cur = head[i]; cur; cur = next[cur])
if (scc[i] != scc[to[cur]])
addEdgeN(scc[i], scc[to[cur]]), in[scc[to[cur]]]++;
int cnt = 0;
for (int i = 1; i <= scccnt; i++)
if (in[i] == 0) cnt++;
for (int i = 1; i <= scccnt; i++)
if (sccsz[i] == 1 && in[i] == 0)
{
bool flag = true;
for (int j = nHead[i]; j; j = nNext[j])
if (in[nTo[j]] <= 1) {
flag = false;
break;
}
if (flag) {
cnt--;
break;
}
}
printf("%.6lf", 1.0 * (n - cnt) / n);
return 0;
}