POJ3259——Bellman_foed—

POJ3259——Bellman_foed——Wormholes

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: A single integer, F. F farm descriptions follow.
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

Lines 1..F: For each farm, output "YES" if FJ can achieve his goal, otherwise output "NO" (do not include the quotes).

Sample Input

Sample Output

NO
YES

Hint

For farm 1, FJ cannot travel back in time.
For farm 2, FJ could travel back in time by the cycle 1->2->3->1, arriving back at his starting location 1 second before he leaves. He could start from anywhere on the cycle to accomplish this.

Source

USACO 2006 December Gold

大意：有F个test，共N个编号，M个正向的u,v,t，即从u到v要花t秒，可双向，W个虫洞反向的，只单向，要花-t秒，问最后能不能到达出发之前。

就有if(d[edge.v]>d[edge.u]+edge.t){

flag = 0;

d[edge.v] = d[edge.u]+edge.t;

}

意为如果虫洞返回后的时间前于出发的时间，那么满足，经过N此循环都是满足的话，那么就是满足的。

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int MAX = 22222222;
int F,N,M,W,cnt;
int d[6000];
struct edge
{
    int u;
    int v;
    int t;
}edge[6000];

bool bellman_ford()
{
    for(int i = 1; i <= N;i++)
        d[i] = MAX;
    d[1] = 0;
    bool flag;
    for(int i = 1; i <= N;i++){
            flag = 1;
       for(int j = 0; j < cnt; j++){
            if(d[edge[j].v]>d[edge[j].u]+edge[j].t){
                    flag = 0;
              d[edge[j].v] = d[edge[j].u]+edge[j].t;
         }
      }
    if(flag)
        return true;
   }
   return false;
}



int main()
{
    int u,v,t;
    scanf("%d",&F);
    while(F--){
            cnt = 0;
            scanf("%d%d%d",&N,&M,&W);
            for(int i = 1; i <= M;i++){
                scanf("%d%d%d",&u,&v,&t);
                edge[cnt].u = u; edge[cnt].v = v; edge[cnt].t = t;
                cnt++;
                edge[cnt].u = v; edge[cnt].v = u; edge[cnt].t = t;
                cnt++;
            }
            for(int i = 1; i <= W;i++){
                    scanf("%d%d%d",&u,&v,&t);
                    edge[cnt].u = u;edge[cnt].v = v;edge[cnt].t = -t;
                    cnt++;
            }
        if(bellman_ford())
            printf("NO
");
        else printf("YES
");
    }
    return 0;
}

View Code