POJ　３６７８

Katu Puzzle

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 7391		Accepted: 2717

Description

Katu Puzzle is presented as a directed graph G(V, E) with each edge e(a, b) labeled by a boolean operator op (one of AND, OR, XOR) and an integer c (0 ≤ c ≤ 1). One Katu is solvable if one can find each vertex V_i a value X_i (0 ≤ X_i ≤ 1) such that for each edge e(a, b) labeled by op and c, the following formula holds:

X_a op X_b = c

The calculating rules are:

AND 0 1 0 0 0 1 0 1

OR 0 1 0 0 1 1 1 1

XOR 0 1 0 0 1 1 1 0

Given a Katu Puzzle, your task is to determine whether it is solvable.

Input

The first line contains two integers N (1 ≤ N ≤ 1000) and M,(0 ≤ M ≤ 1,000,000) indicating the number of vertices and edges.
The following M lines contain three integers a (0 ≤ a < N), b(0 ≤ b < N), c and an operator op each, describing the edges.

Output

Output a line containing "YES" or "NO".

Sample Input

4 4
0 1 1 AND
1 2 1 OR
3 2 0 AND
3 0 0 XOR

Sample Output

YES

Hint

X₀ = 1, X₁ = 1, X₂ = 0, X₃ = 1.

Source

POJ Founder Monthly Contest – 2008.07.27, Dagger

２-sat 

 #include <iostream>
 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <stack>

 using namespace std;

 const int MAX_N = ;
 const int edge = 1e6 + ;
 int first[ * MAX_N],Next[ * edge],v[ * edge];
 int N,M,dfs_clock,scc_cnt;
 int low[ * MAX_N],pre[ * MAX_N],cmp[ * MAX_N];
 stack<int > S;

 void dfs(int u) {
         low[u] = pre[u] = ++dfs_clock;
         S.push(u);
         for(int e = first[u]; e != -; e = Next[e]) {
                 if(!pre[ v[e] ]) {
                         dfs(v[e]);
                         low[u] = min(low[u],low[ v[e] ]);
                 } else {
                         if(!cmp[ v[e] ]) {
                                 low[u] = min(low[u],pre[ v[e] ]);
                         }
                 }
         }

         if(pre[u] == low[u]) {
                 ++scc_cnt;
                 for(;;) {
                         int x = S.top(); S.pop();
                         cmp[x] = scc_cnt;
                         if(x == u) break;
                 }
         }
 }

 void scc() {
         dfs_clock = scc_cnt = ;
         memset(cmp,,sizeof(cmp));
         memset(pre,,sizeof(pre));

         for(int i = ; i <  * N; ++i) if(!pre[i]) dfs(i);
 }

 void add_edge(int id,int u) {
         int e = first[u];
         Next[id] = e;
         first[u] = id;
 }

 bool solve() {
         scc();
         for(int i = ; i < N; ++i) {
                 if(cmp[i] == cmp[N + i]) return false;
         }
         return true;
 }

 void build(int a,int b,int c,char ch[],int &len) {
         if(strcmp(ch,"AND") == ) {
                     if(c == ) {
                             v[len] = b;
                             add_edge(len++,a + N);
                             v[len] = a;
                             add_edge(len++,b + N);
                     } else {
                             v[len] = b + N;
                             add_edge(len++,a + N);
                             v[len] = a + N;
                             add_edge(len++,b + N);
                             v[len] = a + N;
                             add_edge(len++,a);
                             v[len] = b + N;
                             add_edge(len++,b);
                     }
             }
             if(strcmp(ch,"OR") == ) {
                     if(c == ) {
                             v[len] = b;
                             add_edge(len++,a);
                             v[len] = b;
                             add_edge(len++,b + N);
                             v[len] = a;
                             add_edge(len++,b);
                             v[len] = a;
                             add_edge(len++,a + N);
                     } else {
                             v[len] = b + N;
                             add_edge(len++,a);
                             v[len] = a + N;
                             add_edge(len++,b);
                     }
             }
             if(strcmp(ch,"XOR") == ) {
                     if(c == ) {
                             v[len] = b;
                             add_edge(len++,a);
                             v[len] = a + N;
                             add_edge(len++,b + N);
                             v[len] = a;
                             add_edge(len++,b);
                             v[len] = b + N;
                             add_edge(len++,a + N);
                     } else {
                             v[len] = b + N;
                             add_edge(len++,a);
                             v[len] = a + N;
                             add_edge(len++,b);
                             v[len] = b;
                             add_edge(len++,a + N);
                             v[len] = a;
                             add_edge(len++,b + N);
                     }
             }

 }

 int main()
 {
     //freopen("sw.in","r",stdin);
     scanf("%d%d",&N,&M);
     for(int i = ; i <  * N; ++i) first[i] = -;
     int len = ;
     for(int i = ; i <= M; ++i) {
             int a,b,c;
             char ch[];
             scanf("%d%d%d%s",&a,&b,&c,ch);
             build(a,b,c,ch,len);

     }

     printf("%s\n",solve() ? "YES" : "NO");
     //cout << "Hello world!" << endl;
     return ;
 }