NOIP2010關押罪犯 dfs判定二分圖

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<cctype>
#define maxn 200005
using namespace std;

int n,m;
int Prev[maxn],to[maxn],info[20005],cst[maxn],cnt_e,seq[maxn>>1];

void insert(int u,int v,int w){
	cnt_e++;
	Prev[cnt_e]=info[u];info[u]=cnt_e;to[cnt_e]=v;cst[cnt_e]=w;
}

int is[20005],MID;

bool dfs(int now){
	for(int i=info[now];i;i=Prev[i])
		if(cst[i]>=MID){
			if(is[to[i]]){
				if(is[to[i]]==is[now]) return 0;
			}
			else 
			{
				is[to[i]]=is[now]^1;
				if(!dfs(to[i])) return 0;
			}
		}
	return 1;
}

bool check(int Mid){
	MID=Mid;
	memset(is,0,sizeof is);
	for(int i=1;i<=n;i++)
		if(!is[i]){
			is[i]=2;
			if(!dfs(i)) return 0;
		}
	return 1;
}

int main(){
	//freopen("1.in","r",stdin);
	scanf("%d%d",&n,&m);
	
	int u,v,w;
	for(int i=1;i<=m;i++){
		scanf("%d%d%d",&u,&v,&w);
		insert(u,v,w);
		insert(v,u,w);
		seq[i]=w;
	}
	sort(seq+1,seq+m+1);
	
	int L=1,R=m,Mid,ans;
	while(L<R){
		Mid=(L+R)>>1;
		if(check(seq[Mid])) ans=seq[Mid],R=Mid;
		else L=Mid+1;
	}
	
	printf("%d",seq[L-1]);
}

 

也可以用dsu