dfs一遍給每個齒輪隨便標個值看是否矛盾就行了。

#include<iostream> 
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
#define N 1010
#define M 10010
int read()
{
    int x=0,f=1;char c=getchar();
    while (c<'
 
0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
 
int T,n,m,p[N],t;
struct data{int to,nxt;double len;
}edge[M<<1];
const double eps=1E-8;
bool flag[N],ans;
double f[N];
void addedge(int x,int y,double z){t++;edge[t].to=y,edge[t].nxt=p[x],edge[t].len=z;p[x]=t;}
void dfs(int k)
{
    flag[k]=1;
    for (int i=p[k];i;i=edge[i].nxt)
    if (!flag[edge[i].to])
    {
        f[edge[i].to]
 
=f[k]*edge[i].len;
        dfs(edge[i].to);
    }
    else if (fabs(f[k]*edge[i].len-f[edge[i].to])>eps) ans=0;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("bzoj4602.in","r",stdin);
    freopen("bzoj4602.out","w",stdout);
    const char LL[]="%I64d\n";
#else
    const char LL[]="%lld\n";
#endif
    T=read();
    for (int k=1;k<=T;k++)
    {
        n=read(),m=read();ans=1;
        memset(p,0,sizeof(p));t=0;
        for (int i=1;i<=m;i++)
        {
            int x=read(),y=read(),a=read(),b=read();
            addedge(x,y,(double)b/a),addedge(y,x,(double)a/b);
        }
        memset(flag,0,sizeof(flag));
        for (int i=1;i<=n;i++)
        if (!flag[i]) {f[i]=1;dfs(i);}
        printf("Case #%d: ",k);
        if (ans) printf("Yes\n");else printf("No\n");
    }
    return 0;
}