题目来源：HDU 2444 The Accomodation of Students
题意：n个人能否够分成2组 每组的人不能相互认识 就是二分图判定 能够分成2组 每组选一个2个人认识能够去一个双人间 最多能够有几组
思路：二分图判定+最大匹配
#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;
const int maxn = 550;
int vis[maxn];
int y[maxn];
vector <int> G[maxn];
int n, m;
int color[maxn];
bool bipartite(int u)
{
for(int i = 0; i < G[u].size(); i++)
{
int v = G[u][i];
if(color[u] == color[v])
return false;
if(!color[v])
{
color[v] = 3 - color[u];
if(!bipartite(v))
return false;
}
}
return true;
}
bool dfs(int u)
{
for(int i = 0; i < G[u].size(); i++)
{
int v = G[u][i];
if(vis[v] || color[v] == 1)
continue;
vis[v] = true;
if(y[v] == -1 || dfs(y[v]))
{
y[v] = u;
return true;
}
}
return false;
}
int match()
{
int ans = 0;
memset(y, -1, sizeof(y));
for(int i = 1; i <= n; i++)
{
memset(vis, 0, sizeof(vis));
if(color[i] == 1 && dfs(i))
ans++;
}
return ans;
}
int main()
{
//int T;
//scanf("%d", &T);
while(scanf("%d %d", &n, &m) != EOF)
{
for(int i = 0; i <= n; i++)
G[i].clear();
while(m--)
{
int u, v;
scanf("%d %d", &u, &v);
G[u].push_back(v);
G[v].push_back(u);
}
memset(color, 0, sizeof(color));
int flag = 0;
for(int i = 1; i <= n; i++)
if(!color[i])
{
color[i] = 1;
if(!bipartite(i))
{
puts("No");
flag = 1;
break;
}
}
if(flag)
continue;
printf("%d
", match());
}
return 0;
}