计蒜客-2017 ACM-ICPC 亚洲区（乌鲁木齐赛区）网络赛-F-Islands

题解

代码

#include <iostream>
#include <stack>
#include <cstring>
#include <vector>

using namespace std;

const int MAXN = 1e4 + 7;

int n, m;
int low[MAXN];
int pre[MAXN];
int sccno[MAXN];
int in[MAXN];
int out[MAXN];
int dfs_clock, scc_cnt;
stack<int> s;
vector<int> G[MAXN];

void dfs(int u)
{
low[u] = pre[u] = ++dfs_clock;
s.push(u);

for (int i = 0; i < G[u].size(); i++)
{
int v = G[u][i];
if (!pre[v])
{
dfs(v);
low[u] = min(low[v], low[u]);
}
else if (!sccno[v])
{
low[u] = min(low[u], pre[v]);
}
}

int x;
if (low[u] == pre[u])
{
scc_cnt++;
while (1)
{
x = s.top();
s.pop();
sccno[x] = scc_cnt;
if (x == u)
{
break;
}
}
}
}

int main()
{
int T;
scanf("%d", &T);

while (T--)
{
scanf("%d%d", &n, &m);

for (int i = 1; i <= n; i++)
{
G[i].clear();
}
memset(in, 0, sizeof(in));
memset(out, 0, sizeof(out));
memset(sccno, 0, sizeof(sccno));
memset(pre, 0, sizeof(pre));
dfs_clock = 0, scc_cnt = 0;

int u, v;
for (int i = 0; i < m; i++)
{
scanf("%d%d", &u, &v);
G[u].push_back(v);
}
for (int i = 1; i <= n; i++)
{
if (!pre[i])
{
dfs(i);
}
}
for (int i = 1; i <= n; i++)
{
for (int j = 0; j < G[i].size(); j++)
{
if (sccno[i] != sccno[G[i][j]])
{
out[sccno[i]]++;
in[sccno[G[i][j]]]++;
}
}
}

int cnt1 = 0, cnt2 = 0;
for (int i = 1; i <= scc_cnt; i++)
{
if (in[i] == 0)
{
cnt1++;
}
if (out[i] == 0)
{
cnt2++;
}
}
if (scc_cnt == 1)
{
puts("0");
}
else
{
printf("%d\n", max(cnt1, cnt2));
}
}

return 0;
}