uva 12167 - Proving Equivalences（强连通）

题目链接：uva 12167 - Proving Equivalences

#include <cstdio>
#include <cstring>
#include <vector>
#include <stack>
#include <algorithm>

using namespace std;

const int maxn = 20005;

int N, M, in[maxn], ot[maxn];
int cntlock, cntscc, pre[maxn], low[maxn], sccno[maxn];
vector<int> G[maxn];
stack<int> S;

void dfs (int u) {
pre[u] = low[u] = ++cntlock;
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[u], low[v]);
} else if (!sccno[v])
low[u] = min(low[u], low[v]);
}

if (low[u] == pre[u]) {
cntscc++;
while (true) {
int x = S.top();
S.pop();
sccno[x] = cntscc;
if (x == u) break;
}
}
}

void findSCC () {
cntscc = cntlock = 0;
memset(pre, 0, sizeof(pre));
memset(sccno, 0, sizeof(sccno));
for (int i = 1; i <= N; i++)
if (!pre[i]) dfs(i);
}

void init () {
scanf("%d%d", &N, &M);
for (int i = 1; i <= N; i++) G[i].clear();

int u, v;
while (M--) {
scanf("%d%d", &u, &v);
G[u].push_back(v);
}

findSCC();
}

int main () {
int cas;
scanf("%d", &cas);
while (cas--) {
init ();

for (int i = 1; i <= cntscc; i++) in[i] = ot[i] = 1;
for (int u = 1; u <= N; u++) {
for (int j = 0; j < G[u].size(); j++) {
int v = G[u][j];
if (sccno[u] != sccno[v])
in[sccno[v]] = ot[sccno[u]] = 0;
}
}
int a = 0, b = 0;
for (int i = 1; i <= cntscc; i++) {
if (in[i]) a++;
if (ot[i]) b++;
}
printf("%d\n", cntscc == 1 ? 0 : max(a, b));
}
return 0;
}