codechef September Challenge 2017 Fill The Matrix

#include<cstdio>
#include<cstring>
#include<algorithm>
const int M=1e6+7;
int read(){
    int ans=0,f=1,c=getchar();
    while(c<‘0‘||c>‘9‘){if(c==‘-‘) f=-1; c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){ans=ans*10+(c-‘0‘); c=getchar();}
    return ans*f;
}
int T,n,q,vis[M],c[M];
int first[M],cnt;
bool f;
struct node{int to,next,s;}e[2*M];
void ins(int a,int b,int s){e[++cnt]=(node){b,first[a],s}; first[a]=cnt;}
void insert(int a,int b,int s){ins(a,b,s); ins(b,a,s);}
void clear(){
    f=true; cnt=1;
    memset(c,0,4*(n+1));
    memset(first,0,4*(n+1));
    memset(vis,0,4*(n+1));
}
void dfs(int x){
    if(!f) return ; vis[x]=1;
    for(int i=first[x];i;i=e[i].next){
        int now=e[i].to;
        if(vis[now]){
            if(e[i].s&&c[now]==c[x]) return void(f=false);
            if(!e[i].s&&c[now]!=c[x]) return void(f=false);
        }
        else c[now]=c[x]^e[i].s,dfs(now);
    }
}
int main(){
    T=read();
    while(T--){
        int x,y,s;
        n=read(); q=read(); clear();
        for(int i=1;i<=q;i++) x=read(),y=read(),s=read(),insert(x,y,s);
        for(int i=1;i<=n;i++)if(!vis[i]) dfs(i);
        if(f) printf("yes\n");
        else printf("no\n");
    }
    return 0;
}