poj2983——差分约束，bellman_ford

poj2983——差分约束，bellman_ford

Is the Information Reliable?

Time Limit: 3000MS		Memory Limit: 131072K
Total Submissions: 11560		Accepted: 3658

Description

The galaxy war between the Empire Draco and the Commonwealth of Zibu broke out 3 years ago. Draco established a line of defense called Grot. Grot is a straight line with N defense stations. Because of the cooperation of the stations, Zibu’s Marine Glory cannot march any further but stay outside the line.

A mystery Information Group X benefits form selling information to both sides of the war. Today you the administrator of Zibu’s Intelligence Department got a piece of information about Grot’s defense stations’ arrangement from Information Group X. Your task is to determine whether the information is reliable.

The information consists of M tips. Each tip is either precise or vague.

Precise tip is in the form of P A B X, means defense station A is X light-years north of defense station B.

Vague tip is in the form of V A B, means defense station A is in the north of defense station B, at least 1 light-year, but the precise distance is unknown.

Input

There are several test cases in the input. Each test case starts with two integers N (0 < N ≤ 1000) and M (1 ≤ M ≤ 100000).The next M line each describe a tip, either in precise form or vague form.

Output

Output one line for each test case in the input. Output “Reliable” if It is possible to arrange N defense stations satisfying all the M tips, otherwise output “Unreliable”.

Sample Input

3 4

P 1 2 1

P 2 3 1

V 1 3

P 1 3 1

5 5

V 1 2

V 2 3

V 3 4

V 4 5

V 3 5

Sample Output

Unreliable

Reliable
题意：如题
思路;用spfa须增设超级源点，所以又多了N条边，故TLE，而bellman_ford则可以直接处理

#include<iostream>

#include<cstdio>

#include<cstdlib>

#include<cstring>

#include<algorithm>



using namespace std;



const int maxn=10010;

const int INF=(1<<28);



int N,M;

int a,b,x;

struct Edge

{

    int u,v,w;

};Edge edge[maxn*100];int e;

char ch;

int dist[maxn];



void add_edge(int u,int v,int w)

{

    edge[e++]={u,v,w};

}



bool relax(int cur)

{

    int tmp=dist[edge[cur].u]+edge[cur].w;

    if(tmp>dist[edge[cur].v]){

        dist[edge[cur].v]=tmp;

        return true;

    }

    return false;

}



bool bellman_ford()

{

    memset(dist,0,sizeof(dist));

    for(int i=0;i<N;i++){

        bool flag=0;

        for(int j=0;j<e;j++){

            if(relax(j)) flag=1;

        }

        if(!flag) return true;

    }

    for(int i=0;i<e;i++){

        if(relax(i)) return false;

    }

    return true;

}



int main()

{

    while(cin>>N>>M){

        e=0;

        char ch;

        while(M--){

            getchar();

            scanf("%c",&ch);

            if(ch=='P'){

                //cin>>a>>b>>x;

                scanf("%d%d%d",&a,&b,&x);

                add_edge(b,a,x);

                add_edge(a,b,-x);

            }

            else{

                scanf("%d%d",&a,&b);

                add_edge(b,a,1);

            }

        }

        if(bellman_ford()) cout<<"Reliable"<<endl;

        else cout<<"Unreliable"<<endl;

    }

    return 0;

}

View Code