「POJ 1135」Domino Effect（dfs）

#include <cstdio>
#include <cstring>
#include <algorithm>
#define N 100001
#define inf 0x3f3f3f3f
using namespace std;
struct edge{
	int to,next;
	double w;
}e[N<<1];
int head[N],cnt;
void add(int u,int v,int w){
	e[++cnt]=(edge){v,head[u],(double)w};
	head[u]=cnt;
}
int n,m;
int test;
bool one;
int last, second;
double idx[N];
double ans;
void dfs(int x,int fa){
	for(int i=head[x];i;i=e[i].next){
		int v=e[i].to;
		if(v==fa)continue;
		if(idx[x]+e[i].w<idx[v]){
			idx[v]=idx[x]+e[i].w;
			dfs(v,x);
		}
	}
}
int main(){
	while(scanf("%d%d",&n,&m),n||m){
		cnt=1;
		memset(head,0,sizeof head);
		for(int i=1,a,b,l;i<=m;++i){
			scanf("%d%d%d",&a,&b,&l);
			add(a,b,l);add(b,a,l);
		}
		printf("System #%d\n",++test);
		for(int i=2;i<=n;++i)idx[i]=1000000000.0;
		dfs(1,0);
		ans=-1;
		one=true;
		for(int i=1;i<=n;++i){
			if(idx[i]>ans){
				ans=idx[i];
				last=i;
			}
		}
		for(int i=2;i<=cnt;++i){
			double time=(idx[e[i].to]+idx[e[i^1].to]+e[i].w)*1./2;
			if(time>ans){
				ans=time;
				one=false;
				last=e[i].to;second=e[i^1].to;
			}
		}
		printf("The last domino falls after %.1f seconds, ", ans);
		if(one)printf("at key domino %d.\n", last);
		else{
			if(last>second)swap(last,second);
			printf("between key dominoes %d and %d.\n", last, second);
		}
		puts("");
	}
	return 0;
}