[2016-04-03][POJ][3259][Wormholes]

• 时间:2016-04-03 20:22:04 星期日

• 题目编号:[2016-04-03][POJ][3259][Wormholes]

• 题目大意:某农场有n个节点,m条边(双向)和w个虫洞(单向),(走虫洞可以回到过去的时间),问能否和过去的自己相遇

• 分析:

  • 题目意思就是给定一个带负权的图,问是否存在负圈,

spfa_bfs

spfa_dfs

bellman_ford

#include <vector>
#include <queue>
#include <algorithm>
#include <cstring>
#include <cstdio>
using namespace std;
//bfs_spfa
const int maxn = 500 + 10;
struct Edge{
        int v,c;
        Edge(int _v,int _c):v(_v),c(_c){}
};
vector<Edge> e[maxn];
void addedge(int u,int v,int c){
        e[u].push_back(Edge(v,c));
}
bool vis[maxn];
int cnt[maxn],d[maxn];
bool spfa(int s,int n){
        memset(vis,0,sizeof(vis));
        memset(d,0x3f,sizeof(d));
        memset(cnt,0,sizeof(cnt));
        vis[s] = cnt[s] = 1;
        d[s] = 0;
        queue<int> q;
        q.push(s);
        while(!q.empty()){
                int u = q.front();q.pop();
                vis[u] = 0;
                for(int i = 0 ;i < e[u].size();++i){
                        int v = e[u][i].v;
                        if(d[v] > d[u] +  e[u][i].c){
                                d[v] = d[u] +  e[u][i].c;
                                if(!vis[v]){
                                        vis[v] = 1;
                                        q.push(v);
                                        if(++cnt[v] > n)       
                                                return false;
                                }
                        }
                }
        }

        return true;

}
inline void ini(int n){//每组数据存图记得初始化
        for(int i = 0; i <= n ; ++i){
                e[i].clear();
        }
}
int main(){

        int f;
        scanf("%d",&f);
        while(f--){
                int n,m,w;
                scanf("%d%d%d",&n,&m,&w);
                ini(n);
                int s,e,t;
                for(int i = 0;i < m ; ++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,t);
                        addedge(e,s,t);
                }
                for(int i = 0;i < w;++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,-t);
                }
                puts(spfa(1,n)?"NO":"YES");
        }
        return 0;
}

#include <vector>
#include <queue>
#include <algorithm>
#include <cstring>
#include <cstdio>
using namespace std;
//dfs_spfa
const int maxn = 500 + 10;
struct Edge{
        int v,c;
        Edge(int _v,int _c):v(_v),c(_c){}
};
vector<Edge> e[maxn];
void addedge(int u,int v,int c){
        e[u].push_back(Edge(v,c));
}
bool vis[maxn];
int d[maxn];
bool spfa(int u){
        vis[u] = 1;
        for(int i = 0 ;i < e[u].size();++i){
                        int v = e[u][i].v;
                        if(d[v] > d[u] +  e[u][i].c){
                                d[v] = d[u] +  e[u][i].c;
                                if(vis[v] || spfa(v)){
                                        return 1;                                       
                                }
                        }
        }
        vis[u] = 0;
        return 0;
}
inline void ini(int n){
        for(int i = 0; i <= n ; ++i){
                e[i].clear();
        }
        memset(vis,0,sizeof(vis));
        memset(d,0x3f,sizeof(d));
        d[1] = 0;       
}
int main(){

        int f;
        scanf("%d",&f);
        while(f--){
                int n,m,w;
                scanf("%d%d%d",&n,&m,&w);
                ini(n);
                int s,e,t;
                for(int i = 0;i < m ; ++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,t);
                        addedge(e,s,t);
                }
                for(int i = 0;i < w;++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,-t);
                }               
                puts(spfa(1)?"YES":"NO");
        }
        return 0;
}

#include <vector>
#include <cstring>
#include <cstdio>
using namespace std;
//bellman_ford
const int maxn = 500 + 10;
struct Edge{
        int u,v,c;
        Edge(int _u,int _v,int _c):u(_u),v(_v),c(_c){}
};
vector<Edge> e;
void addedge(int u,int v,int c){
        e.push_back(Edge(u,v,c));
}
int d[maxn];
bool bellman_ford(int s,int n){
        memset(d,0x3f,sizeof(d));
        d[s] = 0;
        for(int i = 1;i < n ; ++i){
                bool flg = 0;
                for(int j = 0;j < e.size();++j){
                        int u = e[j].u,v = e[j].v,c = e[j].c;
                        if(d[v] > d[u] + c){
                                d[v] = d[u] + c;
                                flg = 1;
                        }
                }
                if(!flg)        return 1;//没有负环
        }
        for(int j = 0;j < e.size();++j){
                int u = e[j].u,v = e[j].v,c = e[j].c;
                if(d[v] > d[u] + c)     return 0;
        }
        return 1;
}
int main(){

        int f;
        scanf("%d",&f);
        while(f--){
                int n,m,w;
                scanf("%d%d%d",&n,&m,&w);
                e.clear();
                int s,e,t;
                for(int i = 0;i < m ; ++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,t);
                        addedge(e,s,t);
                }
                for(int i = 0;i < w;++i){
                        scanf("%d%d%d",&s,&e,&t);
                        addedge(s,e,-t);
                }               
                puts(bellman_ford(1,n)?"NO":"YES");
        }
        return 0;
}

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