poj 2949 Word Rings 参数搜索+负环探测

类似poj 3621。

代码：

//poj 2949
//sep9
#include <iostream>
#include <string>
#include <map>
#include <vector>
#include <cmath>
using namespace std;
const int maxL=1024;
const int maxM=100100;
char s[maxL];

struct Words
{
	int in,out,len;
}word[maxM];

map< string,int > mymap;
vector<int> ins[maxM],outs[maxM],g[maxM];
double mid,dis[maxM];
int n,cur,vis[maxM];

bool dfs(int u)
{
	vis[u]=cur;
	for(int i=g[u].size()-1;i>=0;--i){
		int v=g[u][i];
		if(dis[u]+mid-word[u].len*1.0<dis[v]){
			dis[v]=dis[u]+mid-word[u].len*1.0;
			if(vis[v]==cur||dfs(v)) return true;
		}
	}
	vis[u]=-1;
	return false;
}

bool neg_circle()
{
	memset(vis,-1,(n+1)*sizeof(vis[0]));
	for(int i=0;i<n;++i) dis[i]=0.0;
	for(cur=0;cur<n;++cur)
		if(dfs(cur)) return true;
	return false;
}

int main()
{
	while(scanf("%d",&n)==1&&n){
		mymap.clear();
		for(int i=0;i<1000;++i){
			ins[i].clear();
			outs[i].clear();
		}
		int t=0;
		for(int i=0;i<n;++i){
			scanf("%s",s);
			word[i].len=strlen(s);
			if(word[i].len<2) continue;
			char tmp[16];
			tmp[0]=s[0],tmp[1]=s[1],tmp[2]='\0';
		 	string x=tmp;
			if(mymap[x]==0) mymap[x]=++t;
 			tmp[0]=s[word[i].len-2],tmp[1]=s[word[i].len-1],tmp[2]='\0';
			string y=tmp;
			if(mymap[y]==0) mymap[y]=++t;
			word[i].in=mymap[x];
			word[i].out=mymap[y];
			ins[word[i].in].push_back(i);
			outs[word[i].out].push_back(i);
		}
		for(int i=0;i<n;++i){
			g[i].clear();
			int u=word[i].out;
			for(int j=ins[u].size()-1;j>=0;--j)
				g[i].push_back(ins[u][j]);
		}
		double l=0.0,r=1024.0;
		while(fabs(r-l)>1e-4){
			mid=(l+r)/2;
			if(neg_circle())
				l=mid;
			else
				r=mid;
		}
		if(abs(l)<1.0)
			printf("No solution.\n");
		else
			printf("%.2lf\n",l+1e-9);
	}
}