BZOJ 2199: [Usaco2011 Jan]奶牛议会 [2-SAT 判断解]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int N=2005,M=2e5+5;
typedef long long ll;
inline int read(){
    char c=getchar();int x=0,f=1;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘;c=getchar();}
    return x*f;
}
inline void get(int &x,char &c){
    c=getchar();
    while(c!=‘h‘&&c!=‘m‘) c=getchar();
    x=read()<<1;
}
int n,m,x,y,vx,vy;
char s[3];
struct edge{
    int v,ne;
}e[M];
int h[N],cnt=0;
inline void ins(int u,int v){
    cnt++;
    e[cnt].v=v;e[cnt].ne=h[u];h[u]=cnt;
}
bool mark[N];
bool dfs(int u){
    if(mark[u^1]) return false;
    if(mark[u]) return true;
    mark[u]=1;
    for(int i=h[u];i;i=e[i].ne)
        if(!dfs(e[i].v)) return false;
    return true;
}
bool check(int u){
    memset(mark,0,sizeof(mark));
    return dfs(u);
}
char ans[N];
int main(){
    freopen("in","r",stdin);
    n=read();m=read();
    for(int i=1;i<=m;i++){
        x=read()<<1;scanf("%s",s);vx=s[0]==‘Y‘;
        y=read()<<1;scanf("%s",s);vy=s[0]==‘Y‘;
        x|=vx;y|=vy;
        ins(x^1,y);ins(y^1,x);
    }
    for(int i=1;i<=n;i++){
        int p=check(i<<1),q=check(i<<1|1);
        if(!p&&!q){puts("IMPOSSIBLE");return 0;}
        else if(p&&q) ans[i]=‘?‘;
        else if(q) ans[i]=‘Y‘;
        else ans[i]=‘N‘;
    }
    for(int i=1;i<=n;i++) putchar(ans[i]);
}