HDU 3062 Party 裸 2-sat

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn = 2100;
struct node{
    int v,next;
}edge[maxn*maxn];
int head[maxn],res[1005*1005][2],low[maxn],dfn[maxn],stack[maxn],in[maxn];
int n,m,id,clock,top;

void add_edge(int u,int v){
    edge[id].v = v;edge[id].next = head[u];head[u] = id++;
}
void init(){
    int c1,c2;
    scanf("%d",&m);
    id = 0;memset(head,-1,sizeof(head));
    for( int i = 0; i < m; i++){
        scanf("%d%d%d%d",&res[i][0],&res[i][1],&c1,&c2);
        res[i][0] = res[i][0]*2 + c1;
        res[i][1] = res[i][1]*2 + c2;
        //两个方向必须相反，否则求出的结果会不对
        add_edge(res[i][0],res[i][1]^1);
        add_edge(res[i][1],res[i][0]^1);
    }
}
void tarjan(int u){
    low[u] = dfn[u] = ++clock;
    stack[top++] = u;
    in[u] = 1;
    for(int id = head[u]; id != -1 ; id = edge[id].next){
        int v = edge[id].v;
        if( !dfn[v] ){
            tarjan(v);
            low[u] = low[u] < low[v] ? low[u] : low[v];
        }
        else if( in[v] && low[u] > dfn[v])
        low[u] = dfn[v];
    }
    if( low[u] == dfn[u]){
        do{
            in[stack[--top]] = 0;
        }while( u != stack[top]);
    }
}

int main(){
    //freopen("in.txt","r",stdin);
    int i;
    while( ~scanf("%d",&n)){
        init();
        memset(dfn,0,sizeof(dfn));
        memset(in,0,sizeof(0));
        clock = top =  0;
        for( i = 0; i < n*2 ; i++)
            if( !dfn[i])
                tarjan(i);
        for(  i = 0; i < m; i++)//在同一个强连通分量中的low值相等
            if(low[res[i][0]] == low[res[i][1]])
                break;
        if( i < m )puts("NO");
        else puts("YES");
    }
    return 0;
}