标签：des style color os io for ar art
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
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
练习一下SPFA。
题意：N块地，M条路，W个虫洞。判断有没有可以是时间倒流的路径。
SPFA:判断有没有入队次数超过N的点。Bellman_Ford就直接判断了。
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<limits.h>
#include<queue>
using namespace std;
const int INF=0x3f3f3f3f;
const int maxn=3000*2;
int end[maxn],cost[maxn];
int next[maxn],cnt[maxn],head[maxn];
int t,n,m,w,e;
bool flag;
int d[maxn],visit[maxn];
void add(int u,int v,int w)
{
end[e]=v;
cost[e]=w;
next[e]=head[u];
head[u]=e++;
}
void SPFA()
{
queue<int>q;
memset(visit,0,sizeof(visit));
memset(cnt,0,sizeof(cnt));
memset(d,INF,sizeof(d));
visit[1]=1;
cnt[1]++;
d[1]=0;
q.push(1);
while(!q.empty())
{
int uu=q.front();
q.pop();
visit[uu]=0;
for(int i=head[uu];i!=-1;i=next[i])
{
int vv=end[i];
int ww=cost[i];
if(d[vv]>d[uu]+ww)
{
d[vv]=d[uu]+ww;
if(!visit[vv])
{
visit[vv]=1;
q.push(vv);
if(++cnt[vv]>=n)
{
flag=false;
return ;
}
}
}
}
}
return ;
}
int main()
{
int x,y,z;
scanf("%d",&t);
while(t--)
{
scanf("%d%d%d",&n,&m,&w);
e=0;
memset(head,-1,sizeof(head));
for(int i=0;i<m;i++)
{
scanf("%d%d%d",&x,&y,&z);
add(x,y,z);
add(y,x,z);
}
for(int i=0;i<w;i++)
{
scanf("%d%d%d",&x,&y,&z);
add(x,y,-z);
}
flag=true;
SPFA();
if(!flag) printf("YES\n");
else printf("NO\n");
}
return 0;
}
Bellman_Ford：
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
using namespace std;
#define MAXN 3000*2
#define INF 0xFFFFFFF
int t , n , m, w;
int dis[MAXN];
struct Edge{
int x;
int y;
int value;
}e[MAXN];
bool judge(){
for(int i = 0 ; i < m*2+w ; i++){
if(dis[e[i].y] > dis[e[i].x] + e[i].value)
return false;
}
return true;
}
void Bellman_Ford(){
dis[1] = 0;
for(int i = 2 ; i <= n ; i++)
dis[i] = INF;
for(int i = 1 ; i <= n ; i++){
for(int j = 0 ; j < m*2+w; j++){
if(dis[e[j].y] > dis[e[j].x] + e[j].value)
dis[e[j].y] = dis[e[j].x] + e[j].value;
}
}
if(judge())
printf("NO\n");
else
printf("YES\n");
}
int main()
{
int uu,vv,ww,i;
scanf("%d",&t);
while(t--)
{
scanf("%d%d%d",&n,&m,&w);
for(i=0;i<m*2;)
{
scanf("%d%d%d",&uu,&vv,&ww);
e[i].x=uu;
e[i].y=vv;
e[i++].value=ww;
e[i].x=vv;
e[i].y=uu;
e[i++].value=ww;
}
for(;i<m*2+w;i++)
{
scanf("%d%d%d",&uu,&vv,&ww);
e[i].x=uu;
e[i].y=vv;
e[i].value=-ww;
}
Bellman_Ford();
}
return 0;
}
POJ 3259 Wormholes（SPFA),布布扣,bubuko.com
标签：des style color os io for ar art
原文地址：http://blog.csdn.net/u013582254/article/details/38497503
