强连通分量之于有向图，与并查集之于无向图，在概念上极其相似，都是寻找互相联系的小部分内容。
POJ 2186 Popular Cows
Description
Every cow's dream is to become the most popular cow in the herd. In a herd of N (1 <= N <= 10,000) cows, you are given up to M (1 <= M <= 50,000) ordered pairs of the form (A, B) that tell you that cow A thinks that cow B is popular. Since popularity is transitive, if A thinks B is popular and B thinks C is popular, then A will also think that C is
popular, even if this is not explicitly specified by an ordered pair in the input. Your task is to compute the number of cows that are considered popular by every other cow.
Input
* Line 1: Two space-separated integers, N and M
* Lines 2..1+M: Two space-separated numbers A and B, meaning that A thinks B is popular.
Output
* Line 1: A single integer that is the number of cows who are considered popular by every other cow.
Sample Input
3 3
1 2
2 1
2 3
Sample Output
1
Hint
Cow 3 is the only cow of high popularity.
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思路：
第一道强连通分量题，照着白书写的。
最后判断拓扑排序最后一个强连通分量是否可行看到两种方法，白书是对最后一个连通分量再逆向dfs一遍，验证是否所有点都能到达；看到网上大多的解法是计算每个连通分量的出度，若当且仅当出度为0的只有1个才符合。
代码如下：
main.cpp
#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;
const int maxn = 1e4+5;
vector<int> g[maxn];
vector<int> rg[maxn];
vector<int> vs; // post order
bool used[maxn];
int top[maxn], du[maxn], n, m, k;
void dfs(int u)
{
used[u] = true;
for (int i = 0; i < g[u].size(); i++)
{
int v = g[u][i];
if (!used[v])
{
dfs(v);
}
}
vs.push_back(u);
}
void rdfs(int u, int k)
{
used[u] = 1;
top[u] = k;
for (int i = 0; i < rg[u].size(); i++)
{
int v = rg[u][i];
if (!used[v])
{
rdfs(v, k);
}
}
}
int scc()
{
memset(used, 0, sizeof(used));
vs.clear();
for (int i = 1; i <= n; i++)
{
if (!used[i])
{
dfs(i);
}
}
memset(used, 0, sizeof(used));
k = 0;
for (int i = vs.size()-1; i >= 0; i--)
{
if (!used[vs[i]])
{
rdfs(vs[i], k);
k++;
}
}
return k;
}
int main()
{
while (cin >> n >> m)
{
for (int i = 1; i < maxn; i++)
{
g[i].clear();
rg[i].clear();
}
for (int i = 1; i <= m; i++)
{
int u, v;
scanf("%d%d", &u, &v);
g[u].push_back(v);
rg[v].push_back(u);
}
int num = scc();
int ans = 0, u;
for (int i = 1; i <= n; i++)
{
if (top[i] == num-1)
{
u = i;
ans++;
}
}
// check out whether the last strong componets part of topological sorting
// can be reached by every other vertices
memset(used, 0, sizeof(used));
rdfs(u, 0);
for (int i = 1; i <= n; i++)
{
if (!used[i])
{
ans = 0;
break;
}
}
printf("%d\n", ans);
continue;
// check out whether the vertices has one out degree are only one in the strong componets
memset(du, 0, sizeof(du));
for (int i = 1; i < n; i++)
{
for (int j = 0; j < g[i].size(); j++)
{
if (top[i] != top[g[i][j]])
{
du[top[i]]++;
}
}
}
int sum = 0;
for (int i = 0; i < num; i++)
{
if (du[i] == 0)
{
sum++;
}
}
if (sum == 1)
{
printf("%d\n", ans);
}
else
{
printf("0\n");
}
}
return 0;
}
测试结果如下：
