poj 2983 Is the Information Reliable?(差分约束)

#include<iostream>
#include<cstdio>
using namespace std;

const int INF=0x3f3f3f3f;
const int maxn=1010;
int n,m,e;
int dis[maxn];

void init(){
e=0;
memset(dis,INF,sizeof(dis));
}

struct node{
int u;
int v;
int w;
}edge[maxn*200];

void addEdge(int u,int v,int w){
edge[e].u=u,edge[e].v=v,edge[e].w=w;
e++;
}

bool relax(int x){
if(dis[edge[x].v]>dis[edge[x].u]+edge[x].w){
dis[edge[x].v]=dis[edge[x].u]+edge[x].w;
return true;
}
return false;
}

bool Bellman_Ford(){
dis[1]=0;
for(int i=1;i<=n;i++){
bool flag=false;
for(int j=0;j<e;j++){
if(relax(j))
flag=true;
}
if(!flag) break;
}
for(int j=0;j<e;j++){
if(relax(j)){
return false;
}
}
return true;
}

int main(){
#ifndef ONLINE_JUDGE
freopen("test.in","r",stdin);
freopen("test.out","w",stdout);
#endif
while(~scanf("%d%d",&n,&m)){
init();
char ch;
int a,b,c;
while(m--){
getchar();
scanf("%c",&ch);
if(ch=='P'){
scanf("%d%d%d",&a,&b,&c);
addEdge(a,b,c);
addEdge(b,a,-c);
}
else{
scanf("%d%d",&a,&b);
addEdge(b,a,-1);
}
}
bool flag=Bellman_Ford();
if(flag) puts("Reliable");
else puts("Unreliable");
}
return 0;
}