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.
solution
正解：差分约束
对于相等，我们就将大于等于和小于等于的限制都加上，对于大于我们还是反向连负边，最后判断是否有负环即可，可以考虑用ｄｆｓ版
#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <cmath>
#define RG register
#define il inline
#define iter iterator
#define Max(a,b) ((a)>(b)?(a):(b))
#define Min(a,b) ((a)<(b)?(a):(b))
using namespace std;
const int N=1005,M=200000;
int n,m,nxt[M],to[M],dis[M],num=0,head[N];char s[2];
il void link(int x,int y,int z){
nxt[++num]=head[x];to[num]=y;dis[num]=z;head[x]=num;}
bool flag=1,vis[N];int f[N];
void Clear(){
for(RG int i=0;i<N;i++)head[i]=f[i]=vis[i]=0;
flag=0;num=0;
}
il bool dfs(int x){
vis[x]=1;
for(RG int i=head[x];i;i=nxt[i]){
int u=to[i];
if(f[x]+dis[i]<f[u]){
if(vis[u])return false;
f[u]=f[x]+dis[i];
if(!dfs(u))return false;
}
}
vis[x]=0;
return true;
}
void work()
{
Clear();
int x,y,z;
for(int i=1;i<=m;i++){
scanf("%s%d%d",s,&x,&y);
if(s[0]=='V')
link(y,x,-1);
else{
scanf("%d",&z);
link(x,y,z);link(y,x,-z);
}
}
for(int i=1;i<=n;i++){
if(!dfs(i)){
puts("Unreliable");
return ;
}
}
puts("Reliable");
}
int main(){
while(~scanf("%d%d",&n,&m))work();
return 0;
}