黑白染色判断是否是二分图*封锁阳光大学
1 #include <iostream>
2 #include <algorithm>
3 #include <cstdio>
4 #include <cstring>
5 using namespace std;
6 const int maxn=5100000;
7 int t,tot,head[maxn],vis[maxn],color[maxn];
8 int ans=0,ans1=0,ans2=0;
9 bool flag;
10 struct node{
11 int to,next;
12 }ed[maxn];
13 void add(int u,int v){
14 ed[++tot].to=v;
15 ed[tot].next=head[u];
16 head[u]=tot;
17 }
18 void dfs(int u,int col){
19 if(vis[u]){
20 if(color[u]!=col)flag=false;
21 }
22 vis[u]=true;
23 if (col==1) color[u]=2,ans1++;
24 else if (col==2) color[u]=1,ans2++;
25 for (int i = head[u];i;i=ed[i].next){
26 int to=ed[i].to;
27 if (vis[to]){
28 if (color[to]==color[u]) flag=false;
29 }
30 else dfs(to,color[u]);
31 }
32 }
33 int main(){
34 memset(ed,0,sizeof(ed));
35 memset(vis,0,sizeof(vis));
36 memset(head,0,sizeof(head));
37 memset(color,0,sizeof(color));
38 flag=true;
39 int n,m;
40 scanf ("%d%d",&n,&m);
41 for (int i = 1;i <= m;i++){
42 int x,y;
43 scanf ("%d%d",&x,&y);
44 add(x,y);add(y,x);
45 }
46 for (int i = 1;i <= n;i++){
47 if (!vis[i]){
48 ans1=0,ans2=0;
49 dfs(i,1);
50 if (flag) ans+=min(ans1,ans2);
51 if (!flag) {break;}
52 }
53 }
54 if (flag) cout<<ans<<endl;
55 else cout<<"Impossible"<<endl;
56 return 0;
57 }
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