poj 3905 Perfect Election（2-SAT判断简单）

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【题目链接】
http://poj.org/problem?id=3905
【思路】
2-SAT简单的判断题
【代码】
#include<iostream>#include<queue>#include<cstdio>#include<cstring>using namespace std;const int MAXN = 1010;const int VN   = MAXN*2;const int EN   = VN*VN*2;int n, m; struct Edge{    int v, next;};struct Graph{    int size, head[VN];    Edge E[EN];    void init(){size=0; memset(head, -1, sizeof(head)); };    void addEdge(int u, int v){        E[size].v = v;        E[size].next = head[u];        head[u] = size++;    }}g;class Two_Sat{public:    bool check(const Graph& g, const int n){        scc(g, 2*n);        for(int i=0; i<n; ++i)            if(belong[i] == belong[i+n])                return false;        return true;    }private:    void tarjan(const Graph& g, const int u){        int v;         low[u] = DFN[u] = ++idx;        sta[top++] = u;        instack[u] = true;        for(int e=g.head[u]; e!=-1; e=g.E[e].next){            v = g.E[e].v;            if(DFN[v] == -1){                tarjan(g, v);                low[u] = min(low[u], low[v]);            }else if(instack[v]){                low[u] = min(low[u], DFN[v]);            }        }        if(DFN[u] == low[u]){            ++bcnt;            do{                v = sta[--top];                instack[v] = false;                belong[v]  = bcnt;            }while(u != v);        }    }    void scc(const Graph& g, const int n){        bcnt = idx = top = 0;        memset(DFN, -1, sizeof(DFN));        memset(instack, 0, sizeof(instack));        for(int i=0; i<n; ++i)            if(DFN[i] == -1)                tarjan(g, i);    }private:    int idx, top, bcnt;    int DFN[VN], low[VN], belong[VN], sta[VN];    bool instack[VN];}sat;int main(){        int  a, b;    while(~scanf("%d%d", &n, &m)){        g.init();        for(int i=0; i<m; ++i){            scanf("%d%d", &a, &b);            if(a>0 && b>0){ // + +                --a; --b;                g.addEdge(a+n, b);                g.addEdge(b+n, a);            }else if(a>0 && b<0){ // + -                b = -b;                  --a; --b;                g.addEdge(a+n, b+n);                g.addEdge(b, a);            }else if(a<0 && b>0){ // - +                a = -a;                --a; --b;                g.addEdge(a, b);                 g.addEdge(b+n, a+n);            }else if(a<0 && b<0){// - -                a = -a; b = -b;                --a; --b;                g.addEdge(a, b+n);                g.addEdge(b, a+n);            }        }        if(sat.check(g, n)) puts("1");        else puts("0");        }    return 0;}