describe ： Yes n(n<=100) personal , Everyone has one or more friends （ Friendship is mutual ）. Divide them into two groups , Make each group have friends in another group .

Ideas ： The main idea is to find a subgraph so that it is a bipartite graph . Direct use dfs dyeing . It's not actually a bipartite graph , Because each subset of this problem can be connected by edges , As long as the conditions given by the title are met . It's a little simpler than bipartite graphs .

//g++ 4.7.2

```#include <cstdio>
#include <iostream>
#include <cstring>
#include <vector>
using namespace std;
const int M = 100 + 10;
int color[M], vis[M];		//color[i] Represents a node i The color of the ,1 According to black ,2 white
vector<int> G[M];
void dfs(int u)
{
vis[u] = 1;
for (int i = 0; i < G[u].size(); ++i)
{
int v = G[u][i];
if (!vis[v])
{
color[v] = 3 - color[u];
dfs(v);
}
}
}
int main()
{
int n, t;
scanf("%d", &n);
for (int i = 1; i <= n; ++i)
while (scanf("%d", &t) && t)
{
G[i].push_back(t);
}
memset(vis, 0, sizeof(vis));
memset(color, 0, sizeof(color));
for (int i = 1; i <= n; ++i)
if (!vis[i])
{
color[i]=1;					// The starting point of each new connected component should be set to 1
dfs(i);
}
int sum = 0;
for (int i = 1; i <= n; ++i)
if (color[i] == 1)
++sum;
printf("%d\n", sum);
for (int i = 1; i <= n; ++i)
if (color[i] == 1)
printf("%d ", i);
return 0;
}
```

There's another way that's more or less , Look like dfs The actual is not , This problem only needs to dye the adjacent points of each node , You don't need recursion .

```#include <cstdio>
#include <iostream>
#include <cstring>
#include <vector>
using namespace std;
const int M = 100 + 10;
int color[M], vis[M];
vector<int> G[M];
void coloring(int u)
{
vis[u] = 1;
color[u] = 1;
for (int i = 0; i < G[u].size(); ++i)
{
int v = G[u][i];
if (!vis[v])
color[v] = 3 - color[u];
vis[v] = 1;
}
}
int main()
{
int n, t;
scanf("%d", &n);
for (int i = 1; i <= n; ++i)
while (scanf("%d", &t) && t)
{
G[i].push_back(t);
}
memset(vis, 0, sizeof(vis));
memset(color, 0, sizeof(color));
for (int i = 1; i <= n; ++i)
if (!vis[i])
coloring(i);
int sum = 0;
for (int i = 1; i <= n; ++i)
if (color[i] == 1)
++sum;
printf("%d\n", sum);
for (int i = 1; i <= n; ++i)
if (color[i] == 1)
printf("%d ", i);
return 0;
}
```

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