POJ 1236 Network of Schools(强连通分量)

POJ 1236 Network of Schools

题目链接

题意:题意本质上就是,给定一个有向图,问两个问题

1、从哪几个顶点出发,能走全全部点

2、最少连几条边,使得图强连通

思路:

#include <cstdio>
#include <cstring>
#include <vector>
#include <stack>
using namespace std; const int N = 105; int n;
vector<int> g[N]; int pre[N], sccno[N], dfn[N], dfs_clock, sccn;
stack<int> S; void dfs_scc(int u) {
pre[u] = dfn[u] = ++dfs_clock;
S.push(u);
for (int i = 0; i < g[u].size(); i++) {
int v = g[u][i];
if (!pre[v]) {
dfs_scc(v);
dfn[u] = min(dfn[u], dfn[v]);
} else if (!sccno[v]) dfn[u] = min(dfn[u], pre[v]);
}
if (dfn[u] == pre[u]) {
sccn++;
while (1) {
int x = S.top(); S.pop();
sccno[x] = sccn;
if (x == u) break;
}
}
} void find_scc() {
sccn = dfs_clock = 0;
memset(pre, 0, sizeof(pre));
memset(sccno, 0, sizeof(sccno));
for (int i = 1; i <= n; i++)
if (!pre[i]) dfs_scc(i);
} int in[N], out[N]; int main() {
while (~scanf("%d", &n)) {
int v;
for (int i = 1; i <= n; i++) g[i].clear();
int cnt = n;
for (int u = 1; u <= n; u++) {
while (~scanf("%d", &v) && v)
g[u].push_back(v);
}
find_scc();
memset(in, 0, sizeof(in));
memset(out, 0, sizeof(out));
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]]++;
out[sccno[u]]++;
}
}
}
int ins = 0, outs = 0;
for (int i = 1; i <= sccn; i++) {
if (!in[i]) ins++;
if (!out[i]) outs++;
}
int ans1 = ins, ans2 = max(ins, outs);
if (sccn == 1) ans2 = 0;
printf("%d\n%d\n", ans1, ans2);
}
return 0;
}

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