bfs->洛谷P1330 封锁阳光大学

版权声明：欢迎大佬批评指正！O(∩_∩)O https://blog.csdn.net/wyh1618/article/details/88542306

#include<iostream>
#include<cstring>
#include<vector>
#include<queue>
using namespace std;
vector <int> g[1000010];
int vis[1000010];
int a[10];
int col[1000010];
queue <int> que;
void bfs(int x)
{
	while(!que.empty()){
		int tmp = que.front();
		vis[tmp] = 1;
		que.pop();
		for(int i=0;i<g[tmp].size();i++){
			int tmnp = g[tmp][i];
			if(!vis[tmnp]){
				que.push(tmnp);
				vis[tmnp] = 1;
				col[tmnp] = (col[tmp]+1)%2;
				a[col[tmnp]]++;
			}
			else if(col[tmp]==col[tmnp]){
				a[0] = -1;
				a[1] = -1;
				return ;
			}
		}
	}
}
int main()
{
	int n,m;
	cin>>n>>m;
	int aa,bb;
	while(m--){
		cin>>aa>>bb;
		g[aa].push_back(bb);
		g[bb].push_back(aa);
	}
	memset(col,-1,sizeof(col));
	int ans = 0;
	int flag = 0;
	for(int i=1;i<=n;i++){
		if(!vis[i]){
			a[0] = 1;
			a[1] = 0;
			col[i] = 0;
			que.push(i);
			bfs(i);
			if(a[0]==-1||a[1]==-1){
				flag = 1;
			}
			else{
				ans += min(a[0],a[1]);
			}
		}
	}
	if(flag)  
		cout<<"Impossible";
    else 
		cout<<ans;
	return 0;
}