【模板】2-SAT

P4782 【模板】2-SAT 问题

我...忘了输出POSSIBLE

总是会忘else if(inst[v]&&dfn[v]<low[u])是else if不是if！！！

然后tarjan要记得入栈...

 

#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<cmath>
#include<vector>
#include<stack>
#include<algorithm> 
using namespace std;
#define Min(x,y) ((x)<(y)?(x):(y))
const int N=1e6+5,M=1e6+5,inf=0x3f3f3f3f,P=9999973;
int n,m;
template <class t>void rd(t &x){
    x=0;int w=0;char ch=0;
    while(!isdigit(ch)) w|=ch=='-',ch=getchar();
    while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    x=w?-x:x;
}

int head[N],tot=0;
struct edge{int u,v,nxt;}e[M<<1];
void add(int u,int v){
    e[++tot]=(edge){u,v,head[u]},head[u]=tot;
}

stack<int>s;
bool inst[N];
int idx=0,Bcnt=0,dfn[N],low[N],bl[N];
void tarjan(int u){
    dfn[u]=low[u]=++idx;
    s.push(u),inst[u]=1;
    for(int i=head[u],v;i;i=e[i].nxt){
        v=e[i].v;
        if(!dfn[v]) tarjan(v),low[u]=Min(low[u],low[v]);
        else if(inst[v]&&dfn[v]<low[u]) low[u]=dfn[v];
    }
    if(dfn[u]==low[u]){
        int v;
        ++Bcnt;
        do{
            v=s.top(),s.pop();
            inst[v]=0,bl[v]=Bcnt;
        }while(u!=v);
    }
}

int main(){
//    freopen("in.txt","r",stdin);
    rd(n),rd(m);
    for(int i=1,x,xx,y,yy;i<=m;++i){
        rd(x),rd(xx),rd(y),rd(yy);
        x=(x<<1)+xx,y=(y<<1)+yy;
        add(x,y^1),add(y,x^1);
    }
    for(int i=2;i<=(n<<1|1);++i)
    if(!dfn[i]) tarjan(i);
    for(int i=1;i<=n;++i)
    if(bl[i<<1]==bl[i<<1|1])
    {puts("IMPOSSIBLE");return 0;}
    puts("POSSIBLE");
    for(int i=1;i<=n;++i)
    printf("%d ",(bl[i<<1]<bl[i<<1|1]));
    return 0;
}

luoguP4782