D37 2-SAT P3007 [USACO11JAN] The Continental Cowngress G

视频链接：D37 2-SAT P3007 [USACO11JAN] The Continental Cowngress G_哔哩哔哩_bilibili

 

 

P3007 [USACO11JAN] The Continental Cowngress G - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

// O(n*n)
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;const int N=2005;
int n,m;
char s1[10],s2[10];
int dfn[N],low[N],stk[N],scc[N],tim,top,cnt;
int head[N],idx,vis[N];
struct Edge{int to,ne;}e[8005];void add(int x,int y){e[++idx].to=y;e[idx].ne=head[x];head[x]=idx;
}
void tarjan(int x){dfn[x]=low[x]=++tim;stk[++top]=x;for(int i=head[x];i;i=e[i].ne){int y=e[i].to;if(!dfn[y]){ //若y尚未访问
      tarjan(y);low[x]=min(low[x],low[y]);}else if(!scc[y]) //若y已访问且未处理low[x]=min(low[x],dfn[y]);}if(low[x]==dfn[x]){ //若x是SCC的根++cnt;for(int y=-1;y!=x;) scc[y=stk[top--]]=cnt;}
}
void dfs(int x){vis[x]=1;for(int i=head[x];i;i=e[i].ne)if(!vis[e[i].to]) dfs(e[i].to);
}
int check(int x){memset(vis,0,sizeof(vis));dfs(x); //找出以x为起点的链上的点// 如果能从x走到x+n或x-n，则返回 trueif(vis[x]&&vis[x<=n?x+n:x-n]) return 1;return 0;
}
int main(){scanf("%d %d",&n,&m);for(int i,j,a,b;m--;){scanf("%d %s %d %s",&i,s1,&j,s2);a=(s1[0]=='Y'); //N:0,i和Y:1,i+nb=(s2[0]=='Y');add(i+!a*n,j+b*n),add(j+!b*n,i+a*n);}for(int i=1;i<=2*n;++i) if(!dfn[i]) tarjan(i);for(int i=1;i<=n;++i)if(scc[i]==scc[i+n]){puts("IMPOSSIBLE"); return 0;}for(int i=1,x,y;i<=n;++i){x=check(i);   //若i→i+n,则取Yy=check(i+n); //若i+n→i,则取Nif(x&&!y) putchar('Y');else if(!x&&y) putchar('N');else if(!x&&!y) putchar('?');}
}