hdu 3062 2-SAT

 1 #include <cstdio>
 2 #include <cstring>
 3 using namespace std;
 4 
 5 const int N = 20001;
 6 const int M = 2000000;
 7 int s[N];
 8 int head[N];
 9 bool mark[N];
10 int n, m, e, c;
11 
12 struct Edge 
13 {
14     int v, next;
15 } edge[M];
16 
17 void init()
18 {
19     e = 0;
20     memset( mark, false, sizeof(mark) );
21     memset( head, -1, sizeof(head) );
22 }
23 
24 void addEdge( int u, int v )
25 {
26     edge[e].v = v;
27     edge[e].next = head[u];
28     head[u] = e++;
29 }
30 
31 void add_clause( int x, int x_val, int y, int y_val )
32 {
33     x = x * 2 + x_val, y = y * 2 + y_val;
34     addEdge( x ^ 1, y );
35     addEdge( y ^ 1, x );
36 }
37 
38 bool dfs( int x )
39 {
40     if ( mark[x ^ 1] ) return false;
41     if ( mark[x] ) return true;
42     mark[x] = true;
43     s[c++] = x;
44     for ( int i = head[x]; i != -1; i = edge[i].next )
45     {
46         int v = edge[i].v;
47         if ( !dfs(v) ) return false;
48     }
49     return true;
50 }
51 
52 bool solve()
53 {
54     for ( int i = 0; i < 2 * n; i += 2 )
55     {
56         if ( !mark[i] && !mark[i + 1] )
57         {
58             c = 0;
59             if ( !dfs(i) )
60             {
61                 while ( c )
62                 {
63                     mark[s[--c]] = false;
64                 }
65                 if ( !dfs( i + 1 ) )
66                 {
67                     return false;
68                 }
69             }
70         }
71     }
72     return true;
73 }
74 
75 int main ()
76 {
77     while ( scanf("%d", &n) != EOF )
78     {
79         scanf("%d", &m);
80         init();
81         while ( m-- )
82         {
83             int a1, a2, c1, c2;
84             scanf("%d%d%d%d", &a1, &a2, &c1, &c2);
85             add_clause( a1, c1, a2, c2 );
86         }
87         printf("%s\n", solve() ? "YES" : "NO");
88     }
89     return 0;
90 }