首页 > 代码库 > poj 3259 Wormholes 【SPFA&&推断负环】
poj 3259 Wormholes 【SPFA&&推断负环】
| Time Limit: 2000MS | Memory Limit: 65536K | |
| Total Submissions: 36852 | Accepted: 13502 |
Description
While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path that delivers you to its destination at a time that is BEFORE you entered the wormhole! Each of FJ‘s farms comprises N (1 ≤ N ≤ 500) fields conveniently numbered 1..N, M (1 ≤ M ≤ 2500) paths, and W (1 ≤ W ≤ 200) wormholes.
As FJ is an avid time-traveling fan, he wants to do the following: start at some field, travel through some paths and wormholes, and return to the starting field a time before his initial departure. Perhaps he will be able to meet himself :) .
To help FJ find out whether this is possible or not, he will supply you with complete maps to F (1 ≤ F ≤ 5) of his farms. No paths will take longer than 10,000 seconds to travel and no wormhole can bring FJ back in time by more than 10,000 seconds.
Input
Line 1 of each farm: Three space-separated integers respectively: N, M, and W
Lines 2..M+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: a bidirectional path between S and E that requires T seconds to traverse. Two fields might be connected by more than one path.
Lines M+2..M+W+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: A one way path from S to E that also moves the traveler back T seconds.
Output
Sample Input
2 3 3 1 1 2 2 1 3 4 2 3 1 3 1 3 3 2 1 1 2 3 2 3 4 3 1 8
Sample Output
NO YES
Hint
分析：
英语不好。就不在翻译了。
意思就是求最短路。假设最短路中有负环，输出yes，没有输出no。
代码：
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
#include<cstdlib>
#define INF 0x3f3f3f3f
#define maxn 10010
using namespace std;
int N,M,W;
int pnum;
int dis[maxn];
int vis[maxn];
int head[maxn];
int used[maxn];
bool cnt;
struct node{
int from;
int to;
int val;
int next;
};
node pp[2*maxn];
void addpp(int u,int v,int w)
{
node E ={u,v,w,head[u]};
pp[pnum]=E;
head[u]=pnum++;
}
void init()
{
int a,b,c;
while(M--)
{
scanf("%d%d%d",&a,&b,&c);
addpp(a,b,c);
addpp(b,a,c);
}
}
void gmap()
{
int a,b,c;
while(W--)
{
scanf("%d%d%d",&a,&b,&c);
addpp(a,b,-c);
}
}
void SPFA(int s)
{
queue<int>q;
memset(dis,INF,sizeof(dis));
memset(vis,0,sizeof(vis));
memset(used,0,sizeof(used));
dis[s]=0;
vis[s]=1;
used[s]++;
q.push(s);
while(!q.empty())
{
int u=q.front();
q.pop();
vis[u]=0;
for(int i=head[u];i!=-1;i=pp[i].next)
{
int v=pp[i].to;
if(dis[v]>dis[u]+pp[i].val)
{
dis[v]=dis[u]+pp[i].val;
if(!vis[v])
{
vis[v]=1;
q.push(v);
used[v]++;
if(used[v]>N)
{
cnt=true;
return;
}
}
}
}
}
}
int main(){
int T;
scanf("%d",&T);
while(T--)
{
pnum=0;
cnt=false;
memset(head,-1,sizeof(head));
scanf("%d%d%d",&N,&M,&W);
init();
gmap();
SPFA(1);
if(cnt)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}
poj 3259 Wormholes 【SPFA&&推断负环】