POJ 3207 Ikki's Story IV - Panda's Trick

Posted 2020-08-09 ziliuziliu

2-SAT判可行。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxv 1050
#define maxe 200500
using namespace std;
int n,m,a[maxv],b[maxv],g[maxv],nume=0,bel[maxv],dfn[maxv],low[maxv],times=0,cnt=0;
int stack[maxv],top=0;
bool vis[maxv];
struct edge
{
    int v,nxt;
}e[maxe];
void addedge(int u,int v)
{
    e[++nume].v=v;
    e[nume].nxt=g[u];
    g[u]=nume;
}
bool judge(int x,int y)
{
    if ((a[x]<a[y]) && (a[y]<b[x]) && (b[x]<b[y])) return false;
    if ((a[y]<a[x]) && (a[x]<b[y]) && (b[y]<b[x])) return false;
    return true;
}
void tarjan(int x)
{
    dfn[x]=low[x]=++times;stack[++top]=x;vis[x]=true;
    for (int i=g[x];i;i=e[i].nxt)
    {
        int v=e[i].v;
        if (!dfn[v])
        {
            tarjan(v);
            low[x]=min(low[x],low[v]);
        }
        else if (vis[v]) low[x]=min(low[x],dfn[v]);
    }
    if (low[x]==dfn[x])
    {
        cnt++;
        int now=0;
        while (now!=x)
        {
            now=stack[top];top--;
            bel[now]=cnt;vis[now]=false;
        }
    }
}
void get_ans()
{
    for (int i=1;i<=m;i++)
    {
        if (bel[i*2-1]==bel[i*2])
        {
            printf("the evil panda is lying again\n");
            return;
        }
    }
    printf("panda is telling the truth...\n");
}
int main()
{
    scanf("%d%d",&n,&m);
    for (int i=1;i<=m;i++)
    {
        scanf("%d%d",&a[i],&b[i]);
        if (a[i]>b[i]) swap(a[i],b[i]);
    }
    for (int i=1;i<=m;i++)
        for (int j=i+1;j<=m;j++)
        {
            if (!judge(i,j))
            {
                addedge(i*2-1,j*2);addedge(j*2,i*2-1);
                addedge(j*2-1,i*2);addedge(i*2,j*2-1);
            }
        }
    for (int i=1;i<=2*m;i++)
        if (!dfn[i]) tarjan(i);
    get_ans();
    return 0;
}