LuoguP3007 [USACO11JAN] The Continental Cowngress G

#include<bits/stdc++.h>
#define Tp template<typename Ty>
#define Ts template<typename Ty,typename... Ar>
#define W while
#define I inline
#define RI register int
#define LL long long
#define Cn const
#define CI Cn int&
#define gc getchar
#define D isdigit(c=gc())
#define pc(c) putchar((c))
#define min(x,y) ((x)<(y)?(x):(y))
#define max(x,y) ((x)>(y)?(x):(y))
using namespace std;
namespace Debug{
    Tp I void _debug(Cn char* f,Ty t){cerr<<f<<'='<<t<<endl;}
    Ts I void _debug(Cn char* f,Ty x,Ar... y){W(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
    Tp ostream& operator<<(ostream& os,Cn vector<Ty>& V){os<<"[";for(Cn auto& vv:V) os<<vv<<",";os<<"]";return os;}
    #define gdb(...) _debug(#__VA_ARGS__,__VA_ARGS__)
}using namespace Debug;
namespace FastIO{
    Tp I void read(Ty& x){char c;int f=1;x=0;W(!D) f=c^'-'?1:-1;W(x=(x<<3)+(x<<1)+(c&15),D);x*=f;}
    Ts I void read(Ty& x,Ar&... y){read(x),read(y...);}
    Tp I void write(Ty x){x<0&&(pc('-'),x=-x,0),x<10?(pc(x+'0'),0):(write(x/10),pc(x%10+'0'),0);}
    Tp I void writeln(Cn Ty& x){write(x),pc('\n');}
}using namespace FastIO;
Cn int N=1010,M=4010;
int n,m,fir[N<<1],nxt[M<<1],son[M<<1],tot,dfn[N<<1],low[N<<1],stk[N<<1],col[N<<1],top,cnt,cc,vis[N<<1];
I int opp(CI x){return x>n?x-n:x+n;}
I void Add(CI x,CI y){nxt[++tot]=fir[x],fir[x]=tot,son[tot]=y;}
#define to son[i]
I void Tarjan(CI x){
    dfn[x]=low[x]=++cnt,stk[++top]=x;
    RI i;for(i=fir[x];i;i=nxt[i]) if(!dfn[to]) Tarjan(to),low[x]=min(low[x],low[to]);else if(!col[to]) low[x]=min(low[x],dfn[to]);
    if(dfn[x]==low[x]){col[x]=++cc;W(stk[top]^x) col[stk[top--]]=cc;top--;}
}
I void dfs(CI x){
    if(vis[x]) return ;
    vis[x]=1;RI i;for(i=fir[x];i;i=nxt[i]) if(!vis[to]) dfs(to);
}
I int check(CI x){
    RI i;memset(vis,0,sizeof(vis));dfs(x);for(i=1;i<=n;i++) if(vis[i]&&vis[i+n]) return 0;return 1;
}
int main(){
    RI i,x,y;char a,b;for(read(n,m),i=1;i<=m;i++) scanf("%d %c %d %c",&x,&a,&y,&b),x+=(a=='Y')*n,y+=(b=='Y')*n,Add(x,opp(y)),Add(y,opp(x));
    for(i=1;i<=(n<<1);i++) if(!dfn[i]) Tarjan(i);for(i=1;i<=n;i++) if(col[i]==col[i+n]) return puts("IMPOSSIBLE"),0;
    for(i=1;i<=n;i++) x=check(i),y=check(i+n),(x&&y)?(pc('?'),0):x?(pc('Y'),0):(pc('N'),0);
    return 0;
}